Combine similar DrawPaths calls in GrInOrderDrawBuffer

Combines adjacent DrawPaths commands into one single call when a
conservative set of conditions are met. The end result is that whole
paragraphs can be drawn with a single call to
gliStencilThenCoverFillPathInstancedNV(), rather than one call for
each line.

BUG=skia:

Review URL: https://codereview.chromium.org/712223002
4 files changed