Specific textures generation code is quite redundant and not flexible for the user, I'm trying to figure out some easy way to allow raylib users to do the same without needing those functions (very specific and shader dependant). RenderTexture loading and Cubemap textures support must be improved.

Actually, that function should be redesigned...

Problem aligning provided vertex data to multiples of four, because main buffer is intended to bu used with indexed quads... but also shared with triangles and lines.

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...

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...

Why am I still supporting this???

...disposing RenderTexture.

[textures] Added SaveKTX()
[rlgl] Exposed rlGetGlTextureFormats()

When a batch reach its vertex limit, a draw call is issued and batch restarted for refilling but if the draw call was issued for vertex data accumulated inside rlPushMatrix/rlPopMatrix, draw call was issued before the rlPopMatrix, consequently modelview matrix was not properly recovered before the draw call... obviously, it only happened the following draw calls, not the first one...

Now it works ok but this system needs to reviewed, noticed and important frames drop when processing around 20 dynamic batch draw calls, it means filling MAX_QUADS_BATCH (8192) quads of data 20 times per frame, including data updating and sending for draw processing.

Doing some maths, it means:
Vertex data (float) -----> 8192 quads * 4 vertex * 3 comp * 4 byte = 393216 bytes
Texcoords data (float) -> 8192 quads * 4 vertex * 2 comp * 4 byte = 262144 bytes
Color data (uchar) -----> 8192 quads * 4 vertex * 4 comp * 1 byte = 131072 bytes

Thats a total of 786432 bytes (0.75MB) sent to GPU 20 times per frame for processing... I'm testing in an Intel HD Graphics integrated, I imagine is too much data to be sent and and it causes stalls, so the frames drop...

Corrected memory leak!

Still a work in progress but it already works...
Current riqm API could be simplified...

This change allows rlgl.h usage as independent single-file header-only module... still some tweaks required, like removing GLAD dependency... required extensions could be manually loaded!

Also removed shader_distortion.h, embedded in rlgl.h

That way, a user needs only to touch a single file to configure what
features raylib is built with.
Include guards are left out intentionally, because config.h should only
be included in source files, not headers.

Later on, config.h can also define the raylib version (#461).

Added struct Vector4 for convenience

- Renamed enum TextureFormat to PixelFormat for consistency
- Added support for pixel format UNCOMPRESSED_R32
- Using GetPixelDataSize() where required