Remove sqrt() from CheckCollisionSpheres()

Update CheckCollisionSpheres() to avoid sqrt

Square root calls are computationally expensive.  In this case, they can be avoided.  Instead of checking distance<RadA+RadB, check distance squared against (RadA+RadB) squared.  The dot product of Vector3Subtract(B,A) with itself gives distance squared, so I used this code instead of an element-by-element computation of distance squared.  The only downside is that your geometric code is very readable, whereas this is less so.

Required for high resolution timer

Some minor tweaks

Variable operations inside the functions should be evaluated before the function operations.

Variable operations inside the functions should be evaluated before the function operations.

This reverts commit 23c1c0bdb2.

Because add_if_flag_compiles apparently allows unsequenced
even if the compiler doesn't support it..

They not only result in discrepancies between different compiler,
but trigger undefined behavior. Avoid them by having them break
the CMake build and CI.

Otherwise compiling rlgl is too spammy. While at it remove
the now unneeded CheckIncludeFiles.

This fixes the current CI failure.

CI is failing because we still test with GLFW 3.2, while
the internal GLFW is 3.3. To avoid such breakages in future,
build and install our own GLFW instead of relying on an external
Debian package.

We were doing this before, but it was deleted during the
last GLFW update. Readd it to fix the associated macOS CI failure.

Fixes: cd934c9f6 ("Update GLFW to 3.3.1")

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)