but they aren't clipped

Add gimp_display_shell_profile_can_convert_to_u8() which returns
whether the lcms transform can safely write directly into an u8 buffer
without destroying out-of-gammut pixel values, which we assume is the
case for all integer formats. If the function returns FALSE, always
convert via the R'G'B'A float filter_buffer.

Also connect to the image's "precision-changed" signal and update the
profile transform when it's emitted.