rtph26x: Use gst_memory_map() instead of gst_buffer_map()
gst_buffer_map () results in memcopying when a GstBuffer contains more than one GstMemory. 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.
Edited by Ognyan Tonchev