This patch fixes T75938 by setting `useMatcap` for the workbench prepass fragment shader. This results in a correct assignment of `materialData.a = float(gl_FrontFacing);` in [[ https://developer.blender.org/diffusion/B/browse/master/source/blender/draw/engines/workbench/shaders/workbench_prepass_frag.glsl | workbench_prepass_frag.glsl ]] when matcaps are used.