Added low-pass filter stuff. 2D low-pass filtering is working.

The low pass filtering algorithm is working.
However, the blocks that it is filtering are created using white noise. This noise doesn't have the right bandwidth. I'm going to have to generate terrain the same way I did in GridVenture.
create line segments, then split the line segment and add randomness. I'm thinking Gaussian randomness would work the best.
I will need to convert my current 2D terrain generator (line segments at different heights/slopes) into a 3D terrain generator (plane segments at different heights/slopes.
+ ideas.txt
fixed memory hog issues with mapSurface, mapTexture, and filter_lowpass_2D_f().

the following four dynamically malloc-ed variables inside filter_lowpass_2D_f(). were not being properly freed from memory after use:
tempRow1
tempRow2
tempCol1
tempCol2

they are now being properly freed from memory so this program n longer uses gigabytes of data.