support GL_LUMINANCE or GL_{RGB,BGR} input

[headupinclouds]

Currently GL_{RGBA,BGRA} and a GL_NV21 overload for format==0 are supported as ogles_gpgpu inputs. This is suitable for mobile platforms, but desktops can often support other input types, such as GL_LUMINANCE or GL_{RGB,BGR}. These formats have to be supported by converting to one of the two formats above first on the CPU.

TODO: Confirm that loading these formats directly would actually be an optimizations before implementing.