Use references instead of pointers for Metal out params.

Pointers require decorating the variable with a * to read back the
value, which the code generator did not properly handle. There was a
special case to add the * but it only supported assignment into the
variable, not reading back. References require no special decoration.

This change fixes compile errors in Functions.sksl with the "bar"
function. (This test marks `x` as an inout but never actually mutates
it.) It also allows us to remove a special-case workaround for `frexp`,
an intrinsic function which uses a reference for its out-parameter.

Additionally, this CL adds a non-inlining copy of "OutParams.sksl" to
the Metal test directory, as most of our tests which use out-parameters
end up inlining all the code, which hides these sorts of bugs.

Change-Id: I31c4db04f6b512b4cd4fe65b3347b82bdbf039cd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/341000
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
6 files changed