[read-fonts] capture full range of image data in cblc (#863)

These can be sparse which requires reading offset and offset +1 to compute the full range.

ref https://skia-review.googlesource.com/c/skia/+/809056

* remove Option from data_size field

all formats can compute a data size

* add assert message to clarify test failures

font-test-data/src/lib.rs

@@ -38,6 +38,7 @@ pub static CANTARELL_VF_TRIMMED_GLYPHS: &str =
 pub static CHARSTRING_PATH_OPS: &[u8] = include_bytes!("../test_data/ttf/charstring_path_ops.ttf");
 
 pub static EMBEDDED_BITMAPS: &[u8] = include_bytes!("../test_data/ttf/embedded_bitmaps.ttf");
+pub static CBDT: &[u8] = include_bytes!("../test_data/ttf/cbdt.ttf");
 
 pub static HVAR_WITH_TRUNCATED_ADVANCE_INDEX_MAP: &[u8] =
     include_bytes!("../test_data/ttf/hvar_with_truncated_adv_index_map.ttf");