A usecase to demonstrate a possible application of the extended functionality:
**"Rails Overlay that draws only the rails that are on Cobblestone for a subway map."**
With this patch it is very easy to achive that:
```python
MineralOverlay(minerals=[(((0, 0, 0, 66), (0, -1, 0, 4)), (255, 0, 0, 255)),
(((0, 0, 0, 27), (0, -1, 0, 4)), (0, 255, 0, 255))])
```
In this case the overlay will be red for rails on cobblestone and green for powerrails on cobblestone.
The syntax is `(<tuple of conditions>, <target color>)`
* where `<target color>` is a 4 tuple with a `(r, g, b, a)` color
* and `<tuple of conditions>` is a tuple with an arbitrary number of conditions with the following syntax:
`((relx, rely, relz, blkid), ...)` where the `rel<>` parameters specify the relative coordinates to the block that is checked if it matches bklid.
In the example the fist tuple `(0,0,0,66)` checks if at the current position is a
rail while `(0,-1,0,4)` checks if at one below the current position is a cobblestone.
If both are true then the color `(255, 0, 0, 255)` is used.
A Sample Config file exploiting the capabilities:
``` python
worlds['My World'] = "~/.minecraft/saves/test/"
outputdir = "/tmp/test_render"
rendermode = "lighting"
renders["render1"] = {
'world': 'My World',
'title': 'A regular render',
}
renders["render_overlay_dafault_rails"] = {
'world': 'My World',
'title': 'Default Rails',
'rendermode': [ClearBase(), StructureOverlay()],
'overlay': ['render1'],
}
renders["render_overlay_cust_rails"] = {
'world': 'My World',
'title': 'Custom Rails',
#relative coordinates [[(relx, rely, relz, mineral)], (red, green, blue, alpha)]
'rendermode': [ClearBase(), StructureOverlay(structures=[(((0, 0, 0, 66), (0, -1, 0, 4)), (255, 0, 0, 255)),
(((0, 0, 0, 27), (0, -1, 0, 4)), (0, 255, 0, 255))])],
'overlay': ['render1'],
}
```
The "Default Rails" overlay uses default coloring of the structures overlay. "Custom Rails" uses some custom coloring.
fixesoverviewer/Minecraft-Overviewer#556 and fixesoverviewer/Minecraft-Overviewer#787
Would crash instead of gracefully exiting if a tileset dir wasn't provided. Might also change how the memory limit option works - currently has to be provided, even though that's not mentioned.
