gtkglsink: Avoid assertion when applying "auto" rotation method

gtkglsink: Avoid assertion when applying "auto" rotation method

Guard against the orientation not coming from an inexistant tag, nor
from the application (rotation set to "auto") which caused an assertion.

When the application requests the auto rotation method, make sure it is
resolved to a rotation that's applicable.

ERROR:gstreamer/subprojects/gst-plugins-good/ext/gtk/gtkgstglwidget.c:745:gtk_gst_gl_widget_set_rotate_method: code should not be reached

Fixes: 103ceb853a5f7dade07a1ac4aa517e9df6ed5ded
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1134>