Overlap between parts of a map

[IAFAssafB]

Hi!
When taking a large map and dividing it to GRG's , I think it would be useful to have some overlap between the new GRGs. Sometime an important feature of the original map falls right in the stitch of two nearby maps and could get lost.
An overlap of a few dozen pixels/percents might resolve this.
Thanks!

[steven-omaha]

Example:

• !grg h_pages=2 now leads to ltrim=0.5 for one page, and rtrim=0.5 for the other one.
• With the overlap, it would become ltrim=0.475 and rtrim=0.475.

To solve this we need to do the following

1. Add a (for now constant) overlap_factor=0.05, lets the pages overlap by 5%.
2. Adjust ltrim, rtrim, btrim, ttrim in the following manner (shown for horizontal divisions):

x_offset = 1 / args[arg.H_PAGES] * overlap_factor / 2  # overlap gets applied twice 
ltrim = h / args[arg.H_PAGES]  
if h > 0:
     ltrim -= x_offset
rtrim = (1 - (h+1) / args[arg.H_PAGES])
if h < args[arg.H_PAGES] -1:
    rtrim -= x_offset

3. This offsets the grid horizontally. Correct this by passing an x_offset to TeX. This should be easy since the grid is put inside scope of the node which holds the image.
4. Extend the existing grid in positive x-direction for all pages except the last one.
5. Draw a new grid in negative x-direction for all pages except the first one.

For the last two steps we need new arguments for the main function in the TeX package.