The example requires pthreads and now C11 <stdatomic.h>, thus skip it
if either isn't available.

This also fixes the current Travis CI build failure for the
Windows configuration.

A plain variable is insuffecient for inter-thread communication. Both the
compiler and the processor may reorder accesses. The compiler could even
cache dataLoaded with the result that STATE_FINISHED becomes unreachable.

Fix this by using C11 atomic_bool, which guarantees sequential consistency.

This fixes #827.

And while at it, use a switch clause to make the state machine
structure clearer.

Some examples included in this batch require the included libraries: `easings.h` and `raygui.h`. Examples included:

 - shapes_bouncing_ball
 - shapes_collision_area
 - shapes_following_eyes
 - shapes_draw_circle_sector (requires raygui.h)
 - shapes_draw_rectangle_rounded (requires raygui.h)
 - shapes_draw_ring (requires raygui.h)
 - shapes_easings_ball_anim (requires easings.h)
 - shapes_easings_box_anim (requires easings.h)
 - shapes_easings_rectangle_array (requires easings.h)

Add SUPPORT_HIGH_DPI to CMakeOptions.txt

Data loading in a second thread with progress bar in main thread

Added walls collision check

Defined PI (#822)

Added guards to not redefine it if the user is using it with raylib.h
also added an 'f' at the end of the define to keep compliant with raylib's PI define

PI is not always defined in math.h, thus it must be defined in this header

Not sure if already corrected...

This change could break things. So, I created SUPPORT_HIGH_DPI flag to enable it (disabled by default).

Basically, it detects HighDPI display and scales all drawing (and mouse input) appropiately to match the equivalent "standardDPI" screen size on highDPI. It uses screenScaling matrix to do that.

This scaling comes with some undesired effects, like aliasing on default font text (keep in mind that font is pixel-perfect, not intended for any non-rounded scale factor).

The only solution for this aliasing would be some AA postpro filter or implementing the highDPI scaling in a different way: rendering to a texture and scaling it with FILTER_BILINEAR, check `core_window_scale_letterbox.c` example for reference.

Use at your own risk.

Gamepad Rework

Some tweaks on BeginVrDrawing()

Two functions have been renamed for coherence; previous naming was confusing for several users:

 - DrawPolyEx()         ---> DrawTriangleFan()
 - DrawPolyExLines() ---> DrawLineStrip()

Fix Readme

Basically, formating review and some variables naming to follow raylib conventions.

* Working build

* Fix build again, stop deleting files

* Hotfix crash, needs investigating

* Remove VS2015.UWP, I cannot update the project

* Lots of UWP work, added keyboard and mouse press support. Still need to finish scroll wheel, mouse position and cursor hiding, plus other stuff that I haven't seen yet.

* Implemented a ton more things, added BaseApp.h to provide common code to UWP apps.

* Remove constant window dimensions

* Enable and Disable cursor support.

* Actually use mouse delta

* Gamepad Support

* Cleaning and small tweaks

* Restore original example.

* Update comment

* Use 'Messages' to handle the cursor functions so code is more portable.

* Comment

* Comment unused message fields and use vector for mouse pos instead.

* Move messages to utils.h and use messages for everything. No more plat-specific code in raylib.h

* Working build

* Fix build again, stop deleting files

* Hotfix crash, needs investigating

* Remove VS2015.UWP, I cannot update the project

* Lots of UWP work, added keyboard and mouse press support. Still need to finish scroll wheel, mouse position and cursor hiding, plus other stuff that I haven't seen yet.

* Implemented a ton more things, added BaseApp.h to provide common code to UWP apps.

* Remove constant window dimensions

* Enable and Disable cursor support.

* Actually use mouse delta

* Gamepad Support

* Cleaning and small tweaks

* Restore original example.

* Update comment

* Use 'Messages' to handle the cursor functions so code is more portable.

* Comment

* Comment unused message fields and use vector for mouse pos instead.

* Move messages to utils.h and use messages for everything. No more plat-specific code in raylib.h

* Tested some desktop stuff and added projection matrix updates for window resizing.

* Fixed big bad mouse bug

* Fix alt buttons and add hack to combat flickery key presses (far from perfect)

* Remove debug code

* Final commit

* Well, so I thought

* Wow, i am bad

* Remove packages folder

* Remove useless include

* Apply requested changes and fix linux build

* Try to stop packages folder

* Have we fixed the formatting properly?

* Third time's the charm?

* Where did this come from?

* Re-fix

* Autoformat is gonna kill

* Fixed XBOX ONE Support

* Fix tabs

Trying to define a global transformation matrix to scale all content for HiDPI.