OpenGL extensions loading could be improved...

Also reviewed rlCheckBufferLimit()

Now rectangle coordinates refer to upper-left corner instead of bottom-left

Now it should work... but this behaviour could be driver dependant...

Useful in case raylib compilation want to be automated and compilation config flags provided by command line.

It's broken.

To generate 2D polygonal shape

To draw only in defined area of the screen
ADDED: BeginScissorMode(), EndScissorMode()

- Reorganized some variables: Not used on OpenGL 1.1
- Now mipmaps can be generated on OpenGL 1.1 backend
- Renamed whiteTexture to defaultTextureId

This commit implements a big update of rlgl module, intended to optimize some parts. This change could break some code bases... hopefully not, but it could.
The BIG changes to the module are:
 - Replaced LINES-TRIANGLES-QUADS buffers by a single one, now all vertex data is accumulated on a single buffer and managed with registered draw calls. LINES-TRIANGLES-QUADS could be used the same way as before, rlgl will manage them carefully. That's a big improvement of the system.
 - Support multi-buffering if required. Just define MAX_BATCH_BUFFERING desired size (currently set to 1 batch). Should be enough for most of the situations.
 - Removed temporal accumulative buffers for matrix transformations, now transformations are directly applied to vertex when on rlVertex3f()
 - Reviewed rlPushMatrix()/rlPopMatrix() to be consistent with OpenGL 1.1, probably I should remove that ancient behaviour but... well, it was not consistent and now it is.
 - Minor tweaks: LoadText(), I broke it in last update... also multiple comments reviewed.
 - TODO: MAX_BATCH_ELEMENTS checking should probably be reviewed... done some tests and it works but...

stb_truetype supports some .otf fonts, just exposed that functionality.

Support file download to disk from memory filesystem.

Solves audio cracking issues

Using by default memory filesystem (MEMFS), provided by Emscripten