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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

It should be reviewed more carefully...

Requires some work, distortion shader could be move out of raylib to
example code...

Added: DrawRectangleGradientV()
Added: DrawRectangleGradientH()