Fixed|Resources: Stuck loading TEXTUREn lump (reclassifying "custom" resources)

Caused by a refactoring oversight when CompositeTexture::flags()
was changed to return a copy of the value rather than a mutable
reference to the original.
Optimize|Resources: Improved TEXTUREn lump load performance by ~14%

When processing the 'custom' set of definitions for potential
reclassification as original game resources, first construct a name
mapping table for O(log n) lookup performance (using QMultiMap).