Isometric rendering: Reduce safety margin around screen to correct values.

This may be a leftover from the static background rendering, which
has redrawn the background just from time to time, not every frame.



        --------------------------
        |                        |
      . |                        |
     / \|                        |
    <   |>     You can see this  |
     \ /|                        |
      v |                        |
        |                        |
      . --------------------------
     / \
      |
      |
      You still need to draw these, which are mostly off screen.

      To be really safe, you need to find the largest tile of the tile set
      and do the math for that. For simplicity reasons we will treat all
      tiles as if they are as large as the largest tile.

This addresses #1084 and brings down the number of render calls down
to 1300 (2500 before) for a 640x480 window and the grassland tile set.
MapRenderer: remove commented code
Merge pull request #1085 from stefanbeller/master

Address #1084