Add fixed-rate compression mode
While working on fixed-rate compression support in mesa (GL_EXT_texture_storage_compression), I created a modified version of kmscube using the extension to test the feature. Proposing it for inclusion in the main repository.
When mode "compress" is chosen, we request the RGBA texture to be compressed using GL_EXT_texture_storage_compression. The default bitrate is selected (might be "none" as it's up to the driver to decide what should be the default for the given texture.