Simplify GrRTC::clean APIs

The CanClearFullscreen enum type is removed. Most usages of clear() had
kYes because a null scissor rect was provided, or had kNo because the
scissor was really critical to the behavior. A few places did provide a
scissor and kYes (e.g. for initializing the target).

To simplify this, the public GrRTC has two variants of clear(). One with
only a color (for fullscreen clears), and one with a rect for partial
clears. The private API also adds a clearAtLeast() function that replaces
the several cases where we'd have a scissor but could expand to fullscreen.

I find the current control flow in internalClear() to be hard to
follow (albeit I was the one to make it that way...), but later CLs
will improve it.

Bug: skia:10205
Change-Id: I87cf8d688c58fbe58ee854fbc4ffe22482d969c6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/290256
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
28 files changed