d3d12: Get Heaven benchmark running

Fixes #3831 (closed)

For D3D, we map both DATA0 with index 1 and DATA1 to SV_Target1, so we don't need to add a new output for this broken app case. This is the D3D equivalent of the dual_color_blend_by_location config option, but we can just do it generically.

Then the allow_glsl_extension_directive_midshader needs to be used, otherwise shaders fail to compile and the app renders white.