updateAudioContext is almost done

Ease of use considered in api and channels are more convenient as
unsigned char type.

Removed BeginDrawingEx()
Added Begin2dMode() and End2dMode()

Improve DXT-ETC1 support on HTML5

Hiding the struct from user should protect from accidentally modifying
the mix channel. This could cause serious errors down the road.

Look over changes and give feedback please.

There is still an issue where audio will cut off after a brief moment

Boolean errors

This is an early draft, needs lots of work. Still need to figure out way
to calculate total length of song. This is hard because xm tracks stream
out zeros when done, only position in track can be found. Position does
not give any direct value of how much more time is left. I think that by
setting the loop count to 1 and seeking until the end I can total up the
number of samples and come up with a length.

... easy code porting, transalating them to touches and gestures
internally.

Removed function SetCustomCursor(), it can be managed by the user.

I decided it is redundant and could be confusing (when mixed with 3D
drawing). It's not KISS.

I've added .PHONY targets and fixed "clean" recipe.

Only defaultShader required, set default value for vertex color
attribute if not enabled and fragColor uniform

Now it's possible to render to texture, old postprocessing system will
be removed on next raylib version.

if mesh is not loaded properly it breaks the game!

Finally, physics update is handled in main thread using steps to get
accuracy in collisions detection instead of moving it to a new thread.

Examples are finished as simple and clear as I could. Finally, physac
module is MORE simpler than in the first version, calculation everything
by the same way for both types of physic objects.

I tryed to add rotated physics a couple of times but I didn't get
anything good to get a base to improve it. Maybe for the next version...

No bugs or strange behaviours found during testing.

...despite not being enabled on GLFW3

Redesigned to work as standalone and support fordward-compatible context
(shaders review)