gl/memory: store the internal format as the texture format

gl/memory: store the internal format as the texture format

Instead of having special cases at each GL texture creation, upload,
readback or copy for all non-8-bits-per-components.
Simply store the more specific format and retrieve the generic
component/type tuple from that.

Introduce a helper function for retrieving the generic GL format (RGBA,
RGB, RG, R, L, A) and type (BYTE, SHORT, SHORT_5_6_5) from a sized
GL format enum (RGBA8, RGB565, RG8, etc).