Add getblockat function

TODO.txt

@@ -23,6 +23,10 @@
   - possibility to turn off textures completely
   - how to map textures to blocks?
     - figured out nice procedural mapping in Blender :)
+  - PROBABLY LIKE THIS:
+    - background textures: 128x128 with image-specific 256 color palette,
+      pixel access shouldn't be as time critical here
+    - other textures 64x64 with direct 565 values stored
 - Rendering: two pass: 1st pass renders the scaled down background (possibly not
   rendering floor, can be just pure green for "grass") to prevent overflows,
   second pass renders the tracks (or the nearest part of it).