ogles_gpgpu::TriangleProc efficiency

[headupinclouds]

The separable triangle filter used for gradient magnitude normalization is better than the GaussianProc model (in terms of ACF similarity). This filter should be updated to use the interstitial texel weighting trick that that GaussianProc kernel does (from GPUImage).

In this approach the following 11 tap filter can be achieved with 7 texture lookups:

[1 2 3 4 5 6 7 6 5 4 3 2 1] = [{1 2} {3 4} {5 6} 7 {6 5} {4 3} {2 1}]

[headupinclouds]

Fixed by #78