Merge branch 'blender2.8' into greasepencil-object

Cleanup: use r_ prefix for return args

PyDoc: add gpu.shader

Also some syntax corrections.

Cleanup: style

GPUTexture: Fix problem with glGenerateMipmap

Fix T56789: There was issue with certain driver with glGenerateMipmap and
GPU_DEPTH_COMPONENT24.
In this case we just create a complete texture with mipmaps manually
without downsampling / initializing the data.

GP: Improve errase panel and toolbars

Some names were not clear enough and some parameters were missing.

GP: Some changes to eraser panel

GP: Add pressure option to eraser size

Object Mode: Make Flat object selectable in ortho view ...

... when wireframe is enabled

Object Mode: Draw object center after the floorgrid

Object Mode: Make Flat object outline visible in orthographic view

Motion Paths: interactively update current frame location while dragging.

Fix object motion paths not updating correct after filtering optimization.

Motion Paths: only update once when transforming multiple bones or objects.

Fix OpenColorIO link errors in some Linux builds.

The library can get installed in a lib/static subdirectory, so search there
as well now.

GP: Fix T56802 - Add/Delete Vertex Group crashes

Cloth: Improve UI

This reorganizes the cloth UI, and changes some of the behaviour to be
more reasonable.

Changes included here:
* Reorganized cloth panels
* Improved some tooltips
* Removed `vel_damping` option
* Removed cloth pinning checkbox
* Removed stiffness scaling checkbox
* Separated shrinking from sewing
* Separated self collisions from object collisions

Reviewed By: brecht

Differential Revision: http://developer.blender.org/D3691

GPU module: Initial implementation of the `gpu.shader` submodule.

Differential Revision: https://developer.blender.org/D3688

Depsgraph: Use explicit argument values

Subdiv: Cleanup, variable name spelling