Minor tweaks to HAVE_NATIVE_SCALING code

Minor tweaks to HAVE_NATIVE_SCALING code

This mostly just reindents.
* src/image.c (x_set_image_size): Always define, but to a no-op
if !HAVE_NATIVE_SCALING, to avoid an #ifdef elsewhere.
(x_create_x_image_and_pixmap): Move decl to avoid an #ifdef.
(image_create_x_image_and_pixmap): Move #ifdef outside of call.
* src/xterm.c (x_composite_image): Avoid ‘else #endif’.