blur: Reduce floating-point precision for UV coordinates

While blur rendering is mostly memory-bound, reducing ALU utilization is
still helpful. This doesn't result in any noticeable artifacts in the
blurred image.

When tested with 2 layers of another 6-pass blur implementation, this
saves ~50 µs of rendering time on an Adreno 640 GPU at 1440x3040
resolution.

Change-Id: I09798e0d98665b92771e601704653609a240c065
Signed-off-by: Mohammad Hasan Keramat J <ikeramat@protonmail.com>
Signed-off-by: Simão Gomes Viana <devel@superboring.dev>