Restructure pre-local matrix generation code.
Restructure pre-local matrix generation code.

Previously, `generate_default_before_children_glue_code` would create
a new variable but wouldn't actually return its name. This made it
difficult to use in several ways:
- The caller would need to manually regenerate the variable name
- We may just want the matrix expression, not a whole new variable
  (in the context of a runtime effect, we might not have the luxury of
  freely inserting new statements into the code)

We now have a function `pre_local_matrix_for_entry` which returns the
pre-local matrix expression but does not assign it to a variable at
all. This simplifies our implementation of `emit_glue_code_for_entry`
because we no longer need to recreate the variable name.

Change-Id: Ib5e32db1543b34d154c7df6b31140564f888f584
Bug: skia:13405
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/558298
Reviewed-by: Robert Phillips <95e8ac5504948c7bf91b22c16a8dbb7ae7c66bfd@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia