A friend of mine has made a texture mask importer/exporter.
What this program does is opens up SC2 Maps and allows you to view the texture masks and export them so you can edit them in your favourite image editing software, like Paint, Photoshop, Gimp...
Here's what the program looks like:
With this program you can delete the texture content on the masks and create null terrain!
What do the masks do? They are just an image with 16 shades from black to white, where black means the texture will be fully transparent (i.e., nonexistant) and white means the texture will be fully opaque. Note that the editor makes every mask add up to be a solid white square, but this program makes no effort to check and see if too little or too much texture is anywhere. Here is how a mask looks like.
You can add almost every single shape you can come up with, I've created a few images to show you what you can do: