gstreamer: bufferlist: Fix remove() range end being off by one

Fixes a bug from !1444 (merged)

Currently affects the fmp4 examples, as well as hlscmafsink (first output segment is always cut off).

The end index was being calculated the same way as the start one, which is incorrect. It should be +1'd when range is inclusive and left as-is if it's exclusive, not the other way around. Fixed and added a simple test to verify correctness.