[Fonts] Add CBDT alpha bitmap test font generation

Add infrastructure for generating non-PNG CBDT bitmap test fonts. The
existing exportTtxCbdt() only generates PNG-format (17) bitmaps. This
adds exportTtxCbdtAlpha() which generates TTX source files for the four
non-PNG alpha bitmap formats:

 - Format 1: byte-aligned rows, SmallGlyphMetrics
 - Format 2: bit-aligned (packed), SmallGlyphMetrics
 - Format 6: byte-aligned rows, BigGlyphMetrics
 - Format 7: bit-aligned (packed), BigGlyphMetrics

Bug: 388557912
Change-Id: I8635399ce08a284d61dee29b3e8b7d96025e5b98
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1155916
Reviewed-by: Julia Lavrova <jlavrova@google.com>
Reviewed-by: Dominik Röttsches <drott@google.com>
Commit-Queue: Dominik Röttsches <drott@google.com>