Fix uninitialized values

Cycles: Fix building of CUDA kernels after last commit

Cycles: Deduplicate sample range setting and getting

Cycles: Add generic work item functions for OpenCL

Cycles: Add generic work item functions for CUDA

Cycles: Rendering multiple tiles at once for OpenCL mega kernel

Basically the same as for CUDA. No major performance difference, but in
some cases this is a little faster than single tile rendering.

Add get_ideal_tile_size function to RenderEngine

This allows render engines to specify an ideal tile size to use while
rendering. The function is optional, if provided the returned tile size
will override the scene settings, if not provided the original behavior
will be used.

Cycles: Implement the get_ideal_tile_size function

Since devices will be rendering many tiles at once the tile size chosen
is on the smaller side.

A limited selection of tiles sizes is provided as an option. This may be
removed later, for now its mostly useful for debugging.

Cycles: Rendering multiple tiles at once for CUDA

There's no major performance difference from rendering a single large
tile. However if there are too many tiles (such as if the tiles are
16x16) the overhead of updating tile display can become a bit
noticeable. This should be easy to fix by restructuring the tile update
code a little.

Cycles: Overload DeviceTask::update_progress to take multiple tiles

`update_progress` will only update the tile sample once every second,
which is a problem when there are multiple tiles and calls to this
function. The overload allows for multiple tiles to be updated at once
by getting around the rate limit with a single call.

Cycles: Have CPU devices use new the acquire_tiles function

This is mostly for testing at the moment, as the default work request
sizes should result in only one tile being acquired at once. May need to
tweak the request sizes later, but for now this should do.

Cycles: Add SampleRange struct

This struct will be used to describe the ranges of tile samples passed
to the kernel when rendering multiple tiles samples at once.

Cycles: Add functions to acquire multiple tiles at once

Devices will render multiple smallish tiles (and eventually multiple
samples) at once to keep busy. Using smaller tiles has a few benefits:
there's less of an issue of tiles being cropped at image edges, multiple
devices can share work better, and we may even be able to increase
coherency from rendering multiple samples.