Use push constants cap in Metal.

* Sets the cap and uses it for verifying vertex/fragmentBytes size instead
of a magic number.
* Also makes maxPushConstantsSize unsigned to avoid warnings and
unnecessary casting.

Change-Id: I2aaea853604eb3c652ddbae3adb0168a4e41799a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/324880
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed