Merge branch 'strand_editmode' into strand_gpu

Fix for uninitialized v_mirr variable when using edge strokes with mirroring.

Fix unfreed memory due to incorrect pointer checks.

Handle replacement of the shader cache for strands outside the creation function.

This makes it easier to run preparation code outside the gpu module conditionally.

Merge branch 'master' into strand_gpu

Merge branch 'master' into strand_editmode

Merge branch 'master' into strand_gpu

Merge branch 'master' into strand_editmode

Fix incorrect tangent calculations for the "curl" effect.

The result for tangents under a mix transformation is essentially a mix
of the original and target tangents, plus a shape correction term.

Better fix for uninitialized variables.

The previous one probably only works because there are currently no more
than 3 valid control curves for each fiber, so the 4th one resets the
cumulative values to 0.0.

Fix some uninitialized variables, leading to incorrect tangents for strands shading.

Fix for strand curve interpolation length.

The shader was overshooting the actual strand length by one segment.

Removed unused utility function.

Fix for calculation of the view vector in strand shading.

The product with the model-view matrix must take translation into account.

Augmentation of the strand shader with a debugging shader.

The debug shader shares most code with the regular shader. It uses a
geometry shader pass to create extra vector lines.

Cleanup for nicer code structure in the strand vertex shader.

Use a converter class for handling differences between DNA and BMesh strands data.

This moves most of the data access logic for this data out of the GPU code and into
the respective BKE files. The concept is similar to how DerivedMesh is used in other
GPU code.

Deduplication of strand drawing functions.

The two variants for object and edit mode are the same, except for
setting the edit data in the parameters.

Removed deprecated editmode drawing code.

The only real missing feature from this code is drawing points and
selection state, which can be reimplemented quite easily.

New option for selecting a fiber rendering primitive.

Currently lines and ribbons (triangle strips) are supported.
The ribbon width can be controlled with a parameter. Ribbons are
oriented toward the camera, similar to billboards, but shaded like lines
with the Kajiya shader imitating a reflective tube.