[graphite] Change RenderStep::vertexSkSL return type to std::string

This allows RenderSteps to build vertex programs based on conditional
parameters which may not be known at compile time.

Change-Id: Ib6383e17b994260d2ea7b8afd98f3ff6ee72bff5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/625728
Auto-Submit: Arman Uguray <armansito@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
19 files changed