This basically adds two settings:
- highPrecision at the View level, which controls the bit depth
of the vsm shadow texture used. This affects all shadowmaps.
- elvsm per shadowmap, which enables Exponential Layered VSM.
the main change in the shaders is that we now always output the
"negative" EVSM, even when it's not enabled. If no shadowmap uses
ELVSM, then the texture is stil an RG texture and the calculation is
lost (with some luck culled by the shader compiler), either way it's
not a lot of math and it's done only once per shadow texel.
During the color pass, on the other hand, we compute the "negative"
EVSM only if enabled.
