video_core: Emulate scaled min/max blending

[meh7an]

The problem

On GCN the color blender applies blend factors to min/max operations; Vulkan ignores them by spec. A pipeline blending MIN/MAX with src_factor=SRC_COLOR, dst_factor=DST_COLOR expects min/max(src*src, dst*dst) — squared-domain accumulation — but gets plain min/max(src, dst). This is the case the existing "Unimplemented use of min/max blend op with blend factor not equal to one" warning points at. God of War 3 Remastered builds its screen-space light/shadow accumulation with exactly this state, washing out environment materials and leaving its stochastic shadow dither unresolved on screen.

The fix

Flag the pattern in the graphics pipeline key; the fragment shader squares its RGB output for that attachment at export and the Vulkan blend factors are forced to ONE, so fixed-function min/max produces exactly min/max(src^2, dst^2) like the hardware does. Clear values 0 and 1 are fixed points of the transform, so the attachment stays coherent end to end. Pipelines without this blend state are unaffected.

[raphaelthegreat]

The scaled min/max blending thing I would consider as an acceptable workaround for now since full min/max blending emulation is cumbersome. However the readback thing is a hack and not that clean. The real issue that needs solving is having texture cache properly validate rendering a subrect of an image with same pitch

[raphaelthegreat]

When this is true add a warning log so it can be known when this emulation triggers