...linear and perceptual precision

Under certain circumstances (e.g. the image has no color profile),
GimpLayer's implementation of GimpDrawable::convert_type() didn't have
enough information to do the right color space conversion.

Intead of messing with stuff like "set profile in between doing a and b",
simply add a "src_profile" parameter to GimpDrawable::convert_type() so
the complete color space conversion information is available without
relying on obscure states that could change in the future.

Make sure all callers pass the right src_profile, particularly in
gimp_image_convert_precision(), which also needed fixing.