blur: Only set constant shader uniforms once

The values of the 2D texture sampler uniforms never change, so set them
when initializing the BlurFilter instance instead of updating them every
frame. This reduces clutter in the rendering path and results in a
negligible performance gain from reducing API overhead.

Change-Id: I335ea0dbdd2d4409ea71b58b7c7f8b87b04a18d7
Signed-off-by: Mohammad Hasan Keramat J <ikeramat@protonmail.com>
Signed-off-by: Simão Gomes Viana <devel@superboring.dev>