gl_rasterizer: Workaround invalid zeta clears

Some games (like Xenoblade Chronicles 2) clear both depth and stencil
buffers while there's a depth-only texture attached (e.g. D16 Unorm).
This commit reads the zeta format of the bound surface on
ConfigureFramebuffers and returns if depth and/or stencil attachments
were set. This is ignored on DrawArrays but on Clear it's used to just
clear those attachments, bypassing an OpenGL error.