renderengine: avoid divide-by-zero in shaders

We have

   max(1e-6, color.y)

to make sure the divisor is non-zero.  But when mediump means
half-precision floats, 1e-6 is smaller than the minimum positive
normal number.  The line might have no effect at all.

This is noticed when looking into the referenced bug.

Bug: 116864006
Test: manual
Change-Id: Ic39209e19ceeba6f51688dc9549746bf0f6a98e1
Merged-In: Ic39209e19ceeba6f51688dc9549746bf0f6a98e1