When the resolution does not change, remove backbuffer tile by tile

This change introduces a new 'replace' mode for tile transitions: when the
resolution does not change, which happens when mergeNewParams is called,
the tile will be marked with the .olTileReplace class. If this class sets
the tile's imgDiv display to 'none', the backbuffer for the tile will
immediately be removed when the tile is loaded.
Fixing tests
Merge pull request #811 from ahocevar/mergenewparams

When the resolution does not change, remove backbuffer tile by tile. r=@bartvde