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 would solve 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.
Update CHANGES.md.
Remove TexturePool entirely.
Remove unnecessary check for texture.destroy.
Merge pull request #1575 from AnalyticalGraphicsInc/noTexturePooling

Do not use a texture pool for imagery textures.