Blender Git Loki

Cycles: Implement texture size limit simplify option

Main intention is to give some quick way to control scene's memory
usage by clamping textures which are too big. This is really handy
on the early production stages when you first create really nice
looking hi-res textures and only when it all works and approved
start investing time on optimizing your scene.

This is a new option in Scene Simplify panel and it acts as
following: when texture size is bigger than the given value it'll
be scaled down by half for until it fits into given limit.

There are various possible improvements, such as:

- Use threaded scaling using our own task manager.

This is actually one of the main reasons why image resize is
manually-implemented instead of using OIIO's resize. Other
reason here is that API seems limited to construct 3D texture
description easily.

- Vectorization of uchar4/float4/half4 textures.

- Use something smarter than box filter.

Was playing with some other filters, but not sure they are
really better: they kind of causes more fuzzy edges.

Even with such a TODOs in the code the option is already quite
useful.

Reviewers: brecht

Reviewed By: brecht

Subscribers: jtheninja, Blendify, gregzaal, venomgfx

Differential Revision: https://developer.blender.org/D2362