Allow Blitter::fastClear to directly write to external buffer

Allow Blitter::fastClear to directly write to external buffer

When the external buffer is the one that is dirty, avoid a useless
copy from external to internal before fast clearing the buffer by
clearing the external buffer directly.

Change-Id: I469243ba8a2b8617f9a0f8b6e2a089f667b8ae63
Reviewed-on: https://swiftshader-review.googlesource.com/18033
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>