Pick normals using a 3x3 viewport

[Kurtil]

It seems that partiale derivatives don't work very well on a 1x1 viewport. Indeed, picked normals have a flipped direction sometimes. It may be due to the fact that fragments are computed using 2x2 block, and partiale derivative use this implementation to compute de derivative across neighbouring fragment... Pure supposition but using a 3x3 viewport fix the normal flipped direction issue.

Also, this PR slightly update the lines added for picking (as it is the same for picking normals). Indeed, it adds the _uDrawingBufferSize uniform for all pick renderer + uses it in the clip space remapping function to be more correct.

[ghost]

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