Before this commit, the convolution scaler shader functions were pre-
instantiated in the shader file. For every filter size, a corresponding
function (with the filter size as suffix) had to be present.
Change this, and make the C code emit the necessary bits.
This means the shader code is much reduced. (Although hopefully it
doesn't make shader compilation faster - it would require a really dumb
compiler if it spends its time on dead code.)
It also makes it more flexible, which is the main goal.
The DEF_SCALER0 stuff is needed because the C code writes the header of
the shader, at a point where scaler macros are not defined yet.