RaspiStill: Add option to render preview via GL.

Add RaspiTex.c which demonstrates how the camera preview buffers can be
bound to EGL Images and rendered via Open GLES.

To enable GL rendering add --gl to the command line and select a
GL scene
* square - A simple rotating square
* teapot - The classic teapot demo
* mirror - OpenGL ES 2.X Hall of mirrors example