Support swapping of word pairs on odd rows

[mkst]

From https://www.moria.us/blog/2020/11/n64-part20-tmem-format-and-mip-maps#creating-and-loading-mipmapped-assets:

for odd-numbered rows, swap each pair of 4-byte blocks.

Here's a crappy function I used in conker (not allowed to attach it):

def swap(data, width, height, bpp):
    res = b''

    bytes_per_row = int(width * bpp >> 3)
    for i in range(height):
        row = data[i*bytes_per_row:i*bytes_per_row+bytes_per_row]
        if i % 2 == 1:
            out = b''
            if bpp == 32:
                for j in range(len(row) // 16):
                    out += row[16*j+8:16*j+16] + row[16*j:16*j+8]
            else:
                for j in range(len(row) // 8):
                    out += row[8*j+4:8*j+8] + row[8*j:8*j+4]
            res += out
        else:
            res += row
    return res

Open questions:

• What do we currently do with non-8-byte-length'd rows?
  • Throw an exception?
• Where is the best place to add this logic (after the iterator? as sometimes that's pulling back 1 byte and we need to work on 8 bytes at a time)

Example: