Validate metadata image sizes

[Daft-Freak]

Moves parsing the images up a bit and rejects images that are the wrong size (making the error messages more accurate).

This still has the problem that only 32blit and SDL macOS builds run the tool, so other builds don't validate anything. (Resulting in there being a couple of PicoSystem projects that don't build for 32blit due to some values being too long...)

[coveralls]

Pull Request Test Coverage Report for Build 1869879250

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 22 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+1.4%) to 79.462%

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Files with Coverage Reduction	New Missed Lines	%
ttblit/tool/metadata.py	22	76.92%

Totals
Change from base Build 1756581373:	1.4%
Covered Lines:	1153
Relevant Lines:	1451

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💛 - Coveralls

[Daft-Freak]

Moved some code a bit and made running without --file exit after validation instead of reading the file and doing nothing. Also tests.

I've got this other commit to generate a source file that adds all the metadata to pico builds: Daft-Freak@9f9adc5 (requires this for the custom tags/ids Daft-Freak/32blit-beta@9275948)

[Gadgetoid]

Thank you!

The Pico stuff is interesting - I didn't know there was so much capacity for metadata. Parseable by blithub?

[Daft-Freak]

Added parsing for the extra fields to the metadata bot, so probably. (Though I did have to write a parser instead of just running picotool and grabbing its output...)