rhi: Setting srcAlpha or dstAlpha to a xxxxColor blend factor fails on D3D11

...and only on D3D11 (although Metal has not been tested but based on the docs it should behave similarly to OpenGL and Vulkan)

This would show up with, for example, a CustomMaterial that has its source/destination factors set to one of the Color contants, say OneMinusSrcColor. The value then ends up in both srcColor/srcAlpha (or dstColor/dstAlpha). So far so good.

However, D3D11 explicitly disallows using _COLOR (e.g. D3D11_BLEND_SRC_COLOR) in SrcBlendAlpha and DstBlendAlpha in D3D11_RENDER_TARGET_BLEND_DESC. Which is only bad because everything else accepts the equivalents, presumably mapping internally based on being a color or alpha factor (e.g. see the table in https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/VkBlendFactor.html )

So it's up to the D3D11 backend of QRhi to do some mapping as appropriate. (so that SrcColor maps to D3D11_BLEND_SRC_COLOR for SrcBlend/DstBlend but to D3D11_BLEND_SRC_ALPHA for SrcBlendAlpha/DstBlendAlpha etc.)