Transparent pixels of isometric tiles rendered opaquely

[KhinRunite]

Isometric tiles are not drawn transparently, if they contain parts that should be transparent. To reproduce:

1. Create an isometric board in the editor.
2. Lay some tiles (.tst or .iso) on layer 1.
3. Lay tiles with transparent sections on layer 2, over the tiles on layer 1. Parts of the tiles on layer 1 will still be visible in the board editor.
4. Test run the board. The transparent parts of the tiles on layer 2 are rendered opaquely and the tiles on layer 1 cannot be seen.

[swordmaster2k]

I'm moving this out from the bug fix milestone and into Trans 3.2 Part I. It could possibly require major changes to the graphics rendering subsystem and this would be better suited to an overhaul milestone than the bug fix.