Do not use a texture pool for imagery textures.

In current versions of Chrome, at least, using a common texture pool for both directly-loaded imagery textures (not mipmapped) and for reprojected imagery (mipmapped) causes mipmapping artifacts.  Higher mip levels of some textures end up with old data (ie: textures from other parts of the globe).  Using two texture pools instead of one solves the problem, but since we don't seem to be gaining much by the texture pooling anyway, I'm just ripping it out in the name of simplicity.  The problem does not occur in Firefox, so it may be a Chrome bug, but I'm not sure.