common, canvas_get_jpeg_alpha: let top_down be any value
common, canvas_get_jpeg_alpha: let top_down be any value

Allow top_down flag to have any value, only ASSERT it is positive
when needs to be positive and zero otherwise. Allows older server
bug of sending 4 instead of 1 in top down flag to not affect newer
clients (previous patch fixes server).

virtualAMP/spiceplay,virtualAMP/spiceplay,virtualAMP/spiceplay,virtualAMP/spiceplay