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.

OpenURL() is a function that most games probably will never need.
Wasting 512 bytes to store of a static char to store an the URL is not
wise.

I propose to have it dynamic building the string on the fly.

Corrected bug on ImageDrawRectangleLines()

Some projects need the buffer to not be empited when drawing to the render texture. I would suggest making maybe a variation of the function if you'd like to keep backwards compatibility, maybe like: BeginTextureModeNoClear() ?

REVIEWED: ImageDrawRectangle()
ADDED: ImageDrawRectangleLines()

By default, internal white texture was used to draw most of the shapes; some time ago, support for white font character from default internal font was added. That way, all basic drawing (shapes, text) could be performed without a texture change and in a single drawing pass.

Now, we move a step further and we allow configuring the texture (and rectangle) used to do the shapes drawing.

Wait for input events passively (sleep) instead of polling events actively every frame

- Updated dr_mp3 and implemented it
- Reviewed sampleCount vs frameCount
- Reviewed XM playing (some weird things...)

text:     IsEqualText() -WIP-
audio:  SaveWAV()
audio:  ExportWaveAsCode()
textures:  ExportImageAsCode()

The comment sais it allows max 256 files, but the MAX_DIRECTORY_FILES
define actually is set to 512.

Let's have a seperate if linux block for this. Since we will need to
define MAX_FILEPATH_LENGTH for all other cases. And its more readable
like this.

On Linux systems the length of the path is defined in linux/limits.h as
PATH_MAX.
Lets use that for MAX_FILEPATH_LENGTH, which is a define that we can
find in some comments in the code that is actually never used. Instead
often we see 256 handwritten.
So lets have MAX_FILEPATH_LENGTH as a proper define, being set to
PATH_MAX on Linux and to 256 on Windows systems.