OpenType and many other font formats have the concept of pre-computed
metrics for the union of all glyph bounding boxes. This allows for fast
though course quick rejecting of bounds, since the glyphs themselves may
potentially be quite a bit larger than the EM. With the introduction of
variable fonts OpenType does not vary these bounds, so the bounds are
only valid for the default non-varied font.
As a result the fTop, fBottom, fXMax, and fXMin reported in
SkFontMetrics may be bogus. Since simply always setting them to empty
zeros may be disruptive, provide a way forward for new users to check if
the bounds are valid.
This exposed an issue where SkTextBlobBuilder::TightRunBounds does not
handle SkTextBlob::kRSXform_Positioning, so add a test for that.
Change-Id: I872729e0f16e2a196229f9902addf4b07b461590
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301455
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
fcfd0af9fd