RaspiStill example code for YUV fast paths

Update RaspiStill framework to provide access to the new EGL image
targets that provide a fastpath for mapping the individual YUV planes to
one byte per pixel GL_LUMINANCE buffers. These are still accessed as OES
textures so this is effectively a greyscale image created from a single
plane of a MMAL buffer.

Also added some example code (yuv, sobel) that demonstrates how to use
the new EGL image targets.

Also, add -gc to the command line to capture the GL frame-buffer as a TGA file.

Thanks to Tim Gover