Add buffer API to SkCustomMesh.

SkCustomMesh is a class with factory functions. Vertex and index
data are stored in buffer objects rather than raw pointers. User can
provide offsets into the buffers.

Currently the buffers are always CPU backend and are uploaded to the
GPU on each draw. However, buffer the creation API takes a
GrDirectContext which in the future will be used to create a GPU-backed
buffer specific to the passed context.

Bug: skia:12720
Change-Id: If1bb8110f0f2f219b030f9682ab844f1f2207d9f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/527917
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
31 files changed