gst/alpha/gstalpha.c: Try to skip pixels or areas that are too dark or too bright...

gst/alpha/gstalpha.c: Try to skip pixels or areas that are too dark or too bright for us to do meaningfull color dete...

Original commit message from CVS:
Based on patch by: Sebastian Keller <sebastian-keller at gmx dot de>
* gst/alpha/gstalpha.c: (gst_alpha_class_init), (gst_alpha_init),
(gst_alpha_set_property), (gst_alpha_get_property),
(gst_alpha_chroma_key_ayuv), (gst_alpha_chromakey_row_i420):
Try to skip pixels or areas that are too dark or too bright for us to do
meaningfull color detection.
Added properties to control the sensitivity to light and darkness.
Added some small cleanups. Fixes #512345.