Cache page coverage of texture into a hash map
Test done on Champion of Norrath (paltex + DisablePartialInvalidation)
Profiler:
Self of GSTextureCache::SourceMap::Add 5.39% => 0.23%
Self of GSTextureCache::LookupSource 15.27% => 10.82%
Hard to measure on CoN as it depends on memory transfer. Seem to be 5-10 fps faster.