Fix a memory leak that occures when Uniform Variables are updated

The Merge method called when some uniforms are added or changed
on a shader used to leak at each call. Fix that.

Change-Id: I46879903c2891cdf1dd94247d089621464a6f8db
Merge topic 'fix_uniform_variables_leak' into master

6a6f0f12 Fix a memory leak that occures when Uniform Variables are updated