Blitting from a higher bit depth source to a lower bit depth
destination was not working properly because Vulkan expects
clamping, while the blitter was truncating the values.

From section 19.5. Image Copies with Scaling:
"Signed and unsigned integers are converted by first
 clamping to the representable range of the destination
 format, then casting the value."

This fixes all remaining failures in:
dEQP-VK.api.copy_and_blit.core.blit_image.all_formats.color.*

Bug b/119620767

Change-Id: Ibd1c1e4be4f5107e7d423781b46021f4c5df1070
Reviewed-on: https://swiftshader-review.googlesource.com/c/23368Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>