blur: Improve terminology used in mix shader

This improved terminology helps clarify what each uniform refers to,
which will become more important once we add dithering.

Change-Id: I607a08b63c6370d091412905d32bd7d89ae71769
Signed-off-by: Mohammad Hasan Keramat J <ikeramat@protonmail.com>
Signed-off-by: Simão Gomes Viana <devel@superboring.dev>