Blender Git Loki

Cycles: adaptive subdivision support for panoramic cameras.

Adds the code to get screen size of a point in world space, which is
used for subdividing geometry to the correct level. The approximate
method of treating the point as if it were directly in front of the
camera is used, as panoramic projections can become very distorted
near the edges of an image. This should be fine for most uses.

There is also no support yet for offscreen dicing scale, though
panorama cameras are often used for rendering 360� renders anyway.

Fixes T49254.

Differential Revision: https://developer.blender.org/D2468