Contributed by ProfJski

Now shader uses `fragTexCoord` that are the full screen texture coordinates normalized, instead of `gl_fragCoord`, the unnormalized screen coordinates

Also changed path to shader back to what it was originally.

Now dividing by the zoom instead of multiplying (in the shader), so zoom works as expected. Also zoom increase/decrease is now scaled depending on the current zoom.

Note that building examples for web as they are (no code adaptation for web avoiding while loop) implies using the emterpreter... and that's very slow!

Redesigns, deletes and renames

Also noted authors propertly on contributed examples

Now it's required on Windows if not using a busy wait loop

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

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

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)

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

Added walls collision check

Found some issues when building for web using latest emscripten 1.38.30, traced the error and found that eglGetProcAdress does not return function pointers for VAO functionality, supported by extension.

It requires more investigation but now it works (avoiding VAO usage)