rtpsession: Implement RFC 2762

Submitted by sancane

Link to original bug (#745586)

Description

Created attachment 298511
patch generated using git format-patch

rtpsession declares an array of maps to store srrcs but only the
the key 0 is being used. This patch replaces the array of maps
for just one map and remove useless parameters in rtpsession

Patch 298511, "patch generated using git format-patch":
0001-rtpsession-Do-not-use-an-array-of-maps-if-they-are-n.patch