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.

Removing the call to Alpha(1) and replacing it with an inline definition
produces measurable improvements. Replacing each instance of ZV with
Vec{} further improves things. We keep an inline RGBA because there
are circumstances (mostly when using pictures) where we don't want to
have to set colors to get default behavior.

For a fairly triangle-heavy thing, this reduces time spent in SetLen
from something over 10% of execution time to around 2.5% of execution
time.

remove Text.SetMatrix and Text.SetColorMask, add Text.Draw(target, matrix) and Text.DrawColorMask(target, matrix, mask)