Embedders making use of the embedder API always ended up with a
hardcoded log tag of "flutter". Some embedders may wish to set a
different log tag. For example, the Fuchsia embedder sets their log tag
to a launch URL followed by "flutter".

If unset, we continue to default to "flutter".

Fixes https://github.com/flutter/flutter/issues/79819

Previously messages logged from Dart code (e.g. via the print function)
were handled directly in the engine by platform-specific code. This
factors out a LogMessage(tag, message) callback that embedders can
implement with platform-specific code.

This also eliminates a dependency on platform-specific code in the core,
and provides more flexibility to embedders than the current fallback to
stdout, which can be a problem on platforms without a traditional stdout
or with restrictions on stdout.

Fixes https://github.com/flutter/flutter/issues/79685

Notify Skia that we've updated texture handles within the current
binding such that Skia invalidates any assumptions about previous
context modifications that it had made.

This fixes a texture corruption issue reported in
https://github.com/flutter/flutter/issues/78648

CMAP tables are a component of fonts which map character codes to glyph
indices used in the font. In cases where this is missing for a font that
we attempt to load, there is not much gained by logging this. It's also
a common source of false positive bug reports and bug reports with
misleading symptoms in Flutter, such as
https://github.com/flutter/flutter/issues/78929

Instead of logging the error, simply suppress it, since it's not
actionable by the user.

CMAP Reference:
https://docs.microsoft.com/en-us/typography/opentype/spec/cmap

Account for https://flutter-review.googlesource.com/c/recipes/+/11700.