iris: proper legacy format support

This series properly enables alpha and luminance alpha formats in Iris, which ought to fix rendering in a bunch of games when using nine. We also make legacy formats work for texture buffers (just have to plumb through the swizzle), and then drop PIPE_CAP_BUFFER_SAMPLER_VIEW_RGBA_ONLY, which allows us to expose GL_ARB_texture_buffer_object in legacy contexts. This was also the last piece needed for GL 3.1 with GL_ARB_compatibility so Mesa begins automatically exposing that.

We use R and RG and swizzling wherever possible because the sampler internally expands L/I/A/LA to RGBA, using 4x/2x the memory bandwidth somewhere inside the sampler/EU world, according to the documentation, whereas R/RG with swizzling does not. This is better than using the native formats.