This implements support for OpenGL logic operations by emitting code to read
from the TLB if needed and blending the fragment output accordingly. It is
similar to VC4's blend lowering pass, but exclusive to logic operations, since
blending is otherwise supported in hardware.

The pass doesn't handle MSAA targets yet.

Fixes the following piglit tests:
spec/!opengl 1.0/gl-1.0-logicop/*
spec/!opengl 1.1/gl-1.1-xor
spec/!opengl 1.1/gl-1.1-xor-copypixels

It also fixes text cursor rendering in Libreoffice with the GTK+2 theme, which
is rendered via glamor using the XOR logic operation.

v2: fix checks for allowed variable location and maximum render target (Eric)

Reviewed-by: Eric Anholt <eric@anholt.net>