updateData()'s loops checking gt.Len() turns out to have been costing
significant computation, not least because each call then in turn
called gt.vs.Stride().

The computation including a call to Stride() can't be optimized away
safely because the compiler can't tell that Stride() is effectively
constant, but we know it won't change so we can make a slice pointing
at that part of the array.

CPU time for updateData goes from 26.35% to 18.65% in my test case.

It turns out that affine matrices are much simpler than the 3x3 matrices
they imply, and we can use this to dramatically streamline some code.
For a test program, this was about a 50% gain in frame rate just from
the cost of the applyMatrixAndMask calls in imdraw, which were calling
matrix.Project() many times. Simplifying matrix.Project, alone, got a
nearly 50% frame rate boost!

Also modify pixelgl's SetMatrix to copy the six values of a 3x2
Affine into the corresponding locations of a 3x3 matrix.