blur: Skip processing of alpha channel by swizzling RGB

This reduces ALU utilization by avoiding unnecessary calculations that
are discarded in the final fragment color. This saves ~100 µs in another
6-pass blur implementation.

Change-Id: I4ec24518eefce369dbb592ac03a14caeea86df9a
Signed-off-by: Mohammad Hasan Keramat J <ikeramat@protonmail.com>
Signed-off-by: Simão Gomes Viana <devel@superboring.dev>