shared-module/OnDiskBitmap.c: Fix loading small monochrome bitmaps

[SamantazFox]

This PR fixes a bug where OnDiskBitmap was unable to read indexed bitmaps with a small header (e.g BITMAPCOREHEADER or BITMAPINFOHEADER) with a file size inferior to 138 bytes. This is even more obvious with monochrome bitmaps.

I have attached the required code + bitmap to reproduce this issue. Here is the output of the script on a build that doesn't include this PR (warning: french):

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main.py sortie :
Traceback (appels les plus récents en dernier) :
  Fichier "main.py", ligne 17, dans <module>
ValueError: file invalide

cpy_bug_small_bmp_cannnot_load.zip

Technical details

This was caused by the first f_read() call which expected 138 bytes, that is the size of the BMP file header (14 bytes) followed by a BITMAPV5HEADER (124 bytes).

The new code only reads the bare minimum at first: 14 bytes of the file header, plus 12 bytes for a BITMAPCOREHEADER (the smallest possible). If the header size is not 12 bytes, then the code will do another call to f_read() to read the remainder of the header.

Other changes

I removed some intermediary variables (indexed and bits_per_pixel).

I also removed the useless check on number_of_colors, as the code for an indexed bitmap will always compute that value from the number of bits per pixel and this variable is not used outside of the indexed section anyway.

Both of these might save a bit of RAM but more importantly flash size.

[tannewt]

Thank you!