Add global struct definitions to SkSL.

Previously, GLSL and Metal code generators would emit a struct wherever
the type was first used in the code, regardless of where it was
originally defined or what scope the type needs to live in. This CL adds
a ProgramElement for struct definitions, so that structs will now appear
at the top-level as they were originally defined. In the case of Metal,
some special handling is also needed to handle the Globals struct
properly.

Not yet fully supported:
- No special handling for structs declared inside functions yet
- No support for structs in separate scopes with overlapping names
The severity of the remaining issues depends mostly on whether we want
to support structs inside functions in Runtime Effects.

Change-Id: Ia95d4529506cb3fa6da63f5cb548199a93e1c0c5
Bug: skia:10922, skia:10923, skia:10925, skia:10926
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/338600
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
33 files changed