Reject Slugs that have creationMatrix with perspective

This shouldn't happen during normal use [1] but if the data
is corrupted, there are some assumptions that can can cause
issues, like the ones linked in the bug.

This rejects those and turns one assert into an actual runtime
check to provide defense in depth.

[1] https://github.com/google/skia/blob/9eecbdc30f7da675edab96974b23174a9d521e0c/src/text/gpu/SubRunContainer.cpp#L1578-L1581

Bug: 520113415
Fixed: 520113415
Change-Id: I6ec23df9a23ea588fa89f7a62dc1f197fe3905fd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1256016
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Thomas Smith <thomsmit@google.com>