two-step: Always load the BGRT fallback image

two-step: Always load the BGRT fallback image

view_set_bgrt_background() can fail even if the BGRT image was loaded
successfully. So we may need the fallback image even though the
BGRT image was loaded successfully.

This commit also fixes plugin->background_bgrt_fallback_image not being
free-ed and set to NULL when loading the fallback image fails.

Note this also drops the clearing of the use_firmware_background flagss
when we fail to load both the BGRT and the fallback images. Clearing
these flags is not necessary. They are only checked if
plugin->background_bgrt_image or plugin->background_bgrt_fallback_image
are non NULL; and if the loading of both images failed then both
are NULL.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>