blitToVManip: Check out-of-bounds using node position not index (#8127)

blitToVManip: Check out-of-bounds using node position not index (#8127)

Previously, when using 'place on vmanip' to add a schematic to a
lua voxelmanip, if part of the schematic was outside the voxelmanip
volume, the outside part would often appear in a strange place
elsewhere inside the voxelmanip instead of being trimmed off.
This was due to the out-of-bounds check checking the index.

A position outside the voxelmanip can have an index that satisfies
'0 <= index <= voxelmanip volume', causing the node to be placed
at a strange position inside the voxelmanip.

Use 'vm->m_area.contains(pos)' instead.
Move index calculation to later in the code to optimise.