Evas filters: Add optimizable blur function

Evas filters: Add optimizable blur function

Prepare optimization paths for blur operations, as they are VERY
costly. This simple change, when using gcc -O3 flag, boosts
horizontal blur performance by > 50%, because STEP is 1 (and
so, memory accesses, increments, etc... are all very simple)

The objective is to have support for NEON, MMX, SSE, too, with
runtime detection.