Image: Add static is_format_compressed function.

[BlueCube3310]

Adds a new static function to Image for detecting whether a format is compressed, as well as replaces instances of format >= Image::FORMAT_DXT1 and format > Image::FORMAT_RGBE9995 with it across the codebase.

This is done mainly to simplify code, but also for futureproofing in case the Format enum ever gets changed.

[Mickeon]

This is somewhat related to #88669 , since that PR heavily expands upon is_compressed and "asks a question" about the format.
Makes you wonder if it would be better to have those as a static methods, or if exposing is_format_compressed to the public API is even desirable. @Calinou

[clayjohn]

I prefer this PR over #88973. #88973 is totally fine, but I think having the static method is a little nicer as it avoids the problem of these checks getting out of sync

[akien-mga]

Thanks!