meta/blit: Rework how format and aspect choices are made

meta/blit: Rework how format and aspect choices are made

This commit does two things.  First, it introduces choose_* functions for
chosing formats and aspects.  Second, it changes the copy (not blit) code
to use appropreately sized UINT formats for everything except depth.  There
are two main reasons for this:  First, it means that compressed and other
non-renderable texture upload should "just work" because it won't be
tripping over non-renderable formats.  Second, it allows us to easly copy
an RGB buffer to and from an RGBX image because the formats will get
switched over to their UINT variants and the shader will deal with the
extra channel for us.