Check that kernels do not access invalid memory points

[sariyanidi]

The "render" functions such as render_identity_basis_texture do not seem to be checking if a valid point is accessed. We'll probably need to return from the function early in these cases (i.e., when rowix >= N1).

[sariyanidi]

There should be no problem currently, since these kernels are being initialized with the correct dimensions (see kernels render_* in file model_fitter.cu).