layers: Handle memory mapped prior to object binding

layers: Handle memory mapped prior to object binding

This adds initial support for memory that is updated via a MapMemory() call
prior to an object being bound. Initially any mapped allocation will just have
global_valid set for whole allocation. This value is then picked up by any
ranges made off of that allocation.

Really, we need to create new mapped regions and mark them valid, then make
sure that any overlapped objects pick up valid from mapped regions. This
requires more thought and design, though, so commiting this changes as a
simpler and incomplete initial fix.