Add #ifdefs to control loading of YUV conversion shaders

Use defines to enable or avoid loading shaders for I420 and UYVY format
video buffers.
Also cleaned up the shader list by using a defined string literal for
the shader file name suffix for shaders which use particular extensions,
currently those are rectangular textures and Imagination Technologies
streaming texture extensions.
Add IMG streaming texture extension based YUV shaders

These are variants of the existing shaders which convert UYVY using
the Imagination Technologies built in textureStreamIMG function.

Not currently tested.