Export: images may be needlessly re-encoded in some cases

[scurest]

Look at this code for getting the already-encoded data for an image

glTF-Blender-IO/addons/io_scene_gltf2/blender/exp/gltf2_blender_image.py

Lines 192 to 200 in 3bc2858

	if image.source == 'FILE' and image.file_format == self.file_format and \
	not image.is_dirty:
	if image.packed_file is not None:
	data = image.packed_file.data
	else:
	src_path = bpy.path.abspath(image.filepath_raw)
	if os.path.isfile(src_path):
	with open(src_path, 'rb') as f:
	data = f.read()

This assumes image.file_format describes the content of the file on disk/packed file. But that's wrong, image.file_format is only the format to use when saving, cf #986. The file_format check should be removed.

To avoid an unnecessary file read in the case of a file on disk, a check for the file extension could be added.

Re-encoding would be especially bad for JPEGs, since it would be lossy.