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.

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.

Solves audio cracking issues

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

raylib resource data file for Windows platform (containing raylib icon and metadata for executable file) has been renamed to a more consistent name: raylib.rc.data

Also required to work with tcc compiler.

Now `BeginTextureMode()` does not clean buffers automatically, that behaviour has been changed because there could be some case (i.e. painting software) where texture buffer does not ned to be cleared; user is responsible of clearing buffers using `ClearBackground()`

When doing a8dffc63fb I was not aware that
printing a warning and not executing the code would be an option. I only
learned that through 618f220851.

So I propose that we allow all URLs except if the string contains a `'`.
Which could end the URL and call another command via `system()`.

Related to https://github.com/raysan5/raylib/issues/686

See https://github.com/raysan5/raylib/issues/686

Calling just `xdg-open` is not right. One needs to pack the URL in `'`.
If we don't do this then some special characters (like ampersand) will
be executed.

Maybe this is true for Windows and Apple case too, but I don't own any
such system. So please merge this, and if it's true for more cases let's
use `sprintf()` in the other cases too.