Improve error handling in SkSL ByteCode

- Allow for $floatLiteral and $intLiteral when determining type
  categories. These could slip into the IR, leading to asserts.
- We weren't propagating the source text in specialize(), so errors in
  the ByteCodeGenerator would actually assert about a missing fSource.
  It's held by unique_ptr on Program, so we wastefully clone the
  string, but we don't have that many specializations yet, so not too
  bad?

Change-Id: I9c79acfb084e6dc8628625dea039c085ec46dba7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/265598
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed