texelFetch and texelFetchOffset fixes (issue #24)

Take the resource swizzle into account for texelFetch and
texelFetchOffset. texelFetchOffset also had a few other problems
including missing a missing bracket, incorrect arguments and missing bit
casting logic. Improved the loadWithOffset test shader to include more
texture types. Previously it only had a 2D texture, but now it also has
1D, 3D, 1D array and 2D array textures.