icons, fonts.digits: Switch over to 2-bit RLE encoding

The 2-bit RLE encoding, in addition to supporting colour is also fully
ROMable meaning we can save 32 bytes of RAM per image by switching to
2-bit encoding.

Switch everything in icons and font.clock over to 2-bit encoding.

Note: this requires all the clock PNG files to be reencoded (because
      they were originally in 1-bit grayscale format and this is no
      longer supported by the encoder).

This reduces RAM overhead by 480 bytes and has only a negligable effect
on FLASH usage (+4 bytes).

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>

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