Fixes #1376
The headers 'texture_rgb565.h' and 'texture_rgb888' contain template specializations needed for using the 'Texture::rgba' function for the respective pixel formats. The specializations were formerly contained in application-local code.
This patch makes various utilities that were originally developed for the backdrop application publicly available.
The new backdrop found at gems/src/app/backdrop replaces the old program that was hosted in the demo repository.
