Slightly improve cubic chop precision in stroke tessellation shaders

Also adds GrGLSLVertexGeoBuilder::appendRawFunctionDefinition(). This
allows us to define functions in vertex and geometry shaders whose names
don't get mangled.

Bug: skia:10419
Change-Id: I90319b54bcbbb7000c7809cb6ce8d1969a3bd8c5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/327456
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
2 files changed