Round texture bufw down to nearest 16 byte multiple

This is the correct functionality, and what I think JPCSP in fact does.  This also corrects indexed textures, which weren't respecting the previous code properly.

Values of 1, 3, 4, and 7 are identical for a 32-bit texture, clut or otherwise.  It's the same for every type.

@daniel229 let me know if this causes problems for Super Robot Taisen OE.

See also #3239 / #3263.

-[Unknown]