Let the user choose if using rres external library

Updated raylib version to 1.9-dev for development pourposes.

Next raylib version is planned to implement a big amount of changes, so
the version bump.

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

- Added new function: GenImageColor()

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

To be more generic and configurable

Added: DrawRectangleGradientV()
Added: DrawRectangleGradientH()

SetWindowTitle()
GetExtension()

Moved function DrawRectangleT() from text module to shapes
Added some comments

Usefult to draw rectangles using default font texture

Exposed some raymath useful functions to raylib API

Moved PBR material loading to example, right decision?

...to avoid possible conflicting symbols

Namely:
- GenImageHorizontalV
- GenImageHorizontalH
- GenImageChecked
- GenImageWhiteNoise
- GenImageCellular

The gradient implementation may be a bit naive, for example it doesn't do any gamma correction.

useful when using FLAG_WINDOW_RESIZABLE.