This commit should bring the mini_al backend up to feature parity with
the OpenAL backend.

...to define proper values

Compiling GLFW library with raylib avoids external dependencies, this
way we solve version problems in some platforms

Now header is truly multiplatform...

Actually still a small pending check on XBOX gamepad controls that
hopefully will be removed with next GLFW 3.3

Support Universal Windows Platform (UWP):
- Windows 10 App
- Windows Phone
- Xbox One

USE_DEPTH_RENDERBUFFER on OpenGL 2.1

On RPI, after latest drivers update, models were not rendered properly
cause of that...

- Review RPI compilation (core_drop_files not supported)
- Review ImageFormat(), some issues
- GetTextureData() reviewed for RPI

Corrected issue with RPI_CROSS_COMPILE

Improve second option to retrieve texture data from fbo...

Still requires some testing...

...while reviewing GetTextureData() functionality on OpenGL ES 2.0

- Added new function: GenImageColor()

ImageDraw() should be reviewed... specially alpha blending...

To be more generic and configurable

Variable t was read and modified without interleaving sequence points,
technically undefined behavior. Report by Clang's -Wunsequenced