Sequencer: Add gaussian blur effect

Currently this gaussian blur implementation accumulates values in the
square kernel rather that doing X direction and then Y direction because
of the lack of using multiple-staged filters.

Once we can we'll implement a way to apply filter as multiple stages we
can optimize hell of a lot in here.

Another thing we can do is to use SSE2 instructions here.

source/blender/makesrna/intern/rna_sequencer_api.c

@@ -468,6 +468,7 @@ void RNA_api_sequences(BlenderRNA *brna, PropertyRNA *cprop)
 		{SEQ_TYPE_SPEED, "SPEED", 0, "Speed", ""},
 		{SEQ_TYPE_MULTICAM, "MULTICAM", 0, "Multicam Selector", ""},
 		{SEQ_TYPE_ADJUSTMENT, "ADJUSTMENT", 0, "Adjustment Layer", ""},
+		{SEQ_TYPE_GAUSSIAN_BLUR, "GAUSSIAN_BLUR", 0, "Gaussian Blur", ""},
 		{0, NULL, 0, NULL, NULL}
 	};