Provide pixel decoding progress

[fweiss]

Primarily for debugging the "dancing" gif, show how many pixels were decoding for each image data block. Don't just put a printf in the gd code, as that mucks up the test output. Good place to put it is in the gd_main_t or gd_info_t, although the latter is a bit too deep.

• the output length is updated in gd_image_expand_code
• the block type event is GD_BLOCK_IMAGE_DATA, after the call to gd_read_image_data

The spec would go in either:

• image_block_spec.cc
• read_block_error_spec.cc
• read_block_spec.cc

[fweiss]

In image_expand_spec.cc, the expand.outputLength is tested in the spec "full example".

But image_expand_spec.cc does not use gd_main_t.

But need to test new field pixelOutputProgress in gd_main_t.

[fweiss]

Maybe better is read_bblock_spec.cc. It references gd_init() and gd_read_image_data(). However, the latter is currently only checking the correct bytes are read from the mock file, in the "image data" spec.

[fweiss]

Almost fixed, but mushroomed into fixing the error handlers. Could not run the whole 128px gif because the string table ran iut of space for entries and string. That required a major refactor, including tests..