Force input coverage to be only a byte in gpu shaders.

Prime coverage invariant output calculations with single component

BUG=skia:

Review URL: https://codereview.chromium.org/649783003
patch from issue 697743002
working

The key here was to get everything into a single library.  Our shared library build is the easiest way to do that.  Some light skimming online makes it look like perhaps cgo requires shared libraries, so that may be at play here too.

BUG=skia:

Review URL: https://codereview.chromium.org/698943002