rtph26x: Use gst_memory_map() instead of gst_buffer_map() in avc mode

rtph26x: Use gst_memory_map() instead of gst_buffer_map() in avc mode

gst_buffer_map () results in memcopying when a GstBuffer contains
more than one GstMemory and when AVC (length-prefixed) alignment is used.
This has quite an impact on performance on systems with limited amount of
resources. With this patch the whole GstBuffer will not be mapped at once,
instead each individual GstMemory will be iterated and mapped separately.