vtfpp: writing compressed textures at low mips (anything with a dimension lower than 4) is incorrect

[craftablescience]

Compressed textures will pad mips that do not have dimensions that are a multiple of 4 out to be a multiple of 4. This is especially important for the 2x2 and 1x1 mip levels, which should be generated properly.

• Fix calculations of mip size to account for compressed texture padding, and allow going to 1x1 for every format.
• Ensure this is the correct behavior for all types of VTF.

[craftablescience]

Noting that the behavior for reading from specific mip levels seems to be mostly correct, which is why accessing frames and faces after the first present does not break.

[craftablescience]

Fixed in #87