GPUParticles3D: Zero modelview matrix of billboard materials when model matrix is zeroes

[ecmjohnson]

Resolves #72650 by zeroing the modelview matrix of billboards when all scales (X, Y, Z) of the model matrix are exactly zero. This was hypothesized to be the issue by @QbieShay in this comment: the particle gets a zero transform when it is inactive, but since the billboarded material ignores the rotation/scale of the model matrix, this method of preventing them from being rendered was not working.

This implementation introduces a conditional branch to the shader for billboarded materials. An alternative is a non-branching implementation that adds an unconditional 16 multiplications with e.g. the following line:

code += "	MODELVIEW_MATRIX = float(scale_sum != 0.0) * MODELVIEW_MATRIX;\n";

I chose the branching form since it will nearly always be synchronized across the wavefront, particularly in the non-particle use-case where the scales will all be non-zero, and this will most often avoid the cost of the 16 multiplications.

[ecmjohnson]

I made a mistake in my initial implementation as there is no guarantee that the scales will be along the major diagonal of the affine matrix. In fact, completely valid affine transformations can have an affine matrix with a trace of zero.

Instead the check should be against a property of the affine matrix that we know must hold: invertibility. For this it suffices to sum the first column of the affine matrix and check if that is zero. A zero first column cannot be the case for any valid affine transformation.

[ecmjohnson]

PR #75162 resolves the same issue in a cleaner way and in a way that is more in alignment with how "Hide On Contact" collision mode works