Add a variation to mipmap generation perf test to measure performance
when given a format that requires emulation in the Vulkan backend, in
this case GL_RGB.

Bug: angleproject:4551
Change-Id: I4c533d13e35279fec9e007691fb64353735040d7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2259199Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>