This includes improvements on the out-of-gamut colored corner being shown for
unbounded component types out of the [0; 1] range (with some small margin of
error to avoid e.g. a -0.0000001 value to show as out-of-gamut).

There are still improvements to be made on the color rendering. In particular,
it still draws as CAIRO_FORMAT_RGB24 cairo surface. We should probably move to
draw as CAIRO_FORMAT_RGBA128F eventually (more precision and even allowing to
draw unbounded colors with a possible option, instead of always clipping).

Also adding the libgimpwidgets API gimp_widget_get_render_space().