Update shadow render code to use lookup texture

Optimizes performance on low end devices by avoiding exp() in the
fragment shader and lowering the precision of the shadow attributes
used in the shaders.

Test: adb shell su root dumpsys SurfaceFlinger --timestats -dump -maxlayers 0 on wembley
Test: atest SurfaceFlinger_test librenderengine_test
Test: shadows still look like shadows
Change-Id: I0e6e35a2f4f2d31b3126438ebb8b34d92a7d98c0