Make it possible to use the DSL from within the compiler itself

The DSL would not previously function within the compiler, because it
expected to be in charge of everything itself. The compiler and DSL also
disagreed about how to handle some things, such as the DSL not
respecting the compiler's override flags.

This CL moves responsibility for much of the setup process into
DSL::Start(), which the compiler now invokes. DSL::Start() now also
takes a ProgramSettings instead of DSL-specific flags, and the
externalFunctions vector has been moved into ProgramSettings.

Change-Id: I283ed8366e25d67f02c43833743c5f8afdedaefc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/408136
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
19 files changed