Blur shader performance optimizations

- removed mipmap in favor of fbo approach
- bell curve computation outside of shade
- type casting optimizations
- pre-computation of shader variables
- increased UV precision to remove scaling artifacts
- move final copy operation to a blit

Test: visual
Test: systrace
Test: SurfaceFlinger_test
Fixes: 148292484
Change-Id: I2ad38cde837b090cadbadbb79b941dc0524948a4