There's considerable cost associated with creating HWA draw targets
which would negate and regress the performance won by it if used for
too small targets (like small spans of text).
This only uses HWA if >= 8K SurfaceAllocationSize to balance HWA draw
target cost with box blur cost.
Only blur one quadrant of a box-shadow and mirror it to the other quadrants.
This applies only if the corners are symmetrical (square corners or equal
corner radii) otherwise we'll fall back to the old method.
