pan/midgard: Fix depth writeout
This fixes the piglit tests fragdepth_gles2
and glsl-1.10-fragdepth
.
Stencil writeout (e.g. ARB_shader_stencil_export) has not been added back in yet.
SSA_FIXED_REGISTER(1)
is currently used instead of the (branch->src[0] == ~0) ? SSA_FIXED_REGISTER(0) : make_compiler_temp(ctx)
(with the actual register set during RA) used by colour writeout.