Require texture data to be uploaded in a format matching the storage

Require texture data to be uploaded in a format matching the storage

This is almost always the case anyway.  It makes things simpler as I don't
have to figure out what to do if someone tries to upload RGB data into a
LUMINANCE_ALPHA texture, which is really an RG texture in disguise.

There are some potential exceptions, like uploading FLOAT data to a
texture with HALF_FLOAT storage.  I'll write conversion routines if those
become relevant.