Alpha hash support for hair in EEvee

This patch adds support for alpha hash for hair rendering in EEvee. Here's a comparison of with alpha hashing:

{F7588610}

And no alpha hashing:

{F7588615}

Note that this needs "soft shadows" enabled, otherwise shadows will be noisy; here's a render with soft shadows disabled:

{F7588621}

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D5221

Fix crash in opengl render caused by gfx being blacklisted for
non-power-of-2 texture support. Note that all I did was pass
the correct width/height into glReadPixels; the result may not
be the same as if non2 textures were enabled.

Added option in shift-g to select verts by number of connected edges (valence).

Fixed crash in iter api, triggered by calling index_update() on the loop py seq in the py api. Note, I didn't fix the underlying problem (bad iter type) so loops.index_update() still isn't working (it's just not crashing).

Fix related to #30859: bmesh face splitting would set the first loop of the new
faces to a loop quite different than the original first loop. This makes e.g.
duplifaces give rotated results after subdivide or loopcut.

Now it tries to find a first loop that is similar to the old one, to try to keep
the rotations the same.

Changed BM_mesh_bm_from_me to copy customdata layout for empty meshes.

bmesh: fixed crash in bevel

bmesh: fixed merge issues with navmesh (though I've not tested if it works yet).
also fixed a small crasher in bridge.

=bmesh= fixed uv unwrap tool

=bmesh= sculpt without multires updates properly now. not sure I did this correctly, though.