grate: Add cube test program

Simple multi-colored rotated cube using perspective transformation.

Signed-off-by: Thierry Reding <thierry@gilfi.de>
gles: Add support for loading textures from PNG files

This is useful for test programs that want to use texture mapping but
not keep repeating the same code over and over again.

Signed-off-by: Thierry Reding <thierry@gilfi.de>
gles: Add gles-cube-textured test program

Implements a spinning cube using texture mapping instead of uniform
colors.

Signed-off-by: Thierry Reding <thierry@gilfi.de>
gles: Add gles-quad-textured test program

This is similar to the textured cube program, except it renders only one
face.

Signed-off-by: Thierry Reding <thierry@gilfi.de>