Investigate and repair VirtualMem

[meekee7]

Trespasser has a mechanism for managing virtual memory that is used for textures. Unfortunately it is broken.
The symptom of the problem is that Trespasser would always crash with a memory access violation while creating mipmaps while loading a level. If generating mipmaps is disabled, the access violation would occur elsewhere.
PR #76 introduced a workaround for the problem: bypass virtual memory altogether by using regular memory allocation.
The root cause of the problem should be identified and resolved.