renderproto: Make the A4 format optional

This is technically not backwards-compatible, but the only client I can find that actually uses A4 is rendercheck; none of gtk, qt, or libXft will ever use PICT_a4. I don't know of any hardware postdating, say, DirectX 7 that implements A4 directly. At best you might have GL_LUMINANCE4_ALPHA4_EXT and wire luminance to 1.0, but that wins you neither memory, visual quality, nor (probably) performance over A8. None of the open drivers make any attempt to accelerate A4, and I'd be surprised if any of the closed ones did either.

So: slow, ugly, and unused. Let's have none of those.

Signed-off-by: Adam Jackson ajax@redhat.com