While our generic Blitter routine can handle resolving any multisample
format into any other format, the 'fastResolve()' method performs simple
per-component sample averaging, thus assuming the input and output
formats are identical.

This is also demanded by the Vulkan specification:
- vkCmdResolveImage: "srcImage and dstImage must have been created with
  the same image format."
- VkSubpassDescription: "each resolve attachment that is not
  VK_ATTACHMENT_UNUSED must have the same VkFormat as its corresponding
  color attachment."

This change adds an assert which would catch violations of that.

Bug: b/147802090
Change-Id: I23d2d463efbbaed04a782a0cf61b255bf1c25b03
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48088
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>