Sooner filtering for image_downsize()

[anatolbroder]

In the function image_downsize() we have an initialization block with two non trivial time-consuming procedures inside.

	$img_url = wp_get_attachment_url($id);
	$meta = wp_get_attachment_metadata($id);

The filter image_downsize is below the block. But the filter parameters don’t interact with the initialization block. If we move the filter before the initialization block, plugins using the filter will save time.

[SergeyBiryukov]

Introduced in [6952] along with the function itself.

[SergeyBiryukov]

In 23410:

Return from image_downsize() earlier if a custom resize function is used. props anatolbroder. fixes #23392.