vkCmdClearColorImage() and vkCmdClearDepthStencilImage() are currently
implemented using Blitter::clear(), which delegates to fastClear() for
certain formats, or falls back to using a generic Reactor routine.

This change benchmarks the performance for:
 - VK_FORMAT_R8G8B8A8_UNORM (fast path)
 - VK_FORMAT_R32_SFLOAT (slow path)
 - VK_FORMAT_D32_SFLOAT (slow path)

Bug: b/158231104
Change-Id: I5383b322e3ea5de6b21113843fb665a22a92b9bf
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45470
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>