Alloc a single array for all planar stuff.

[toots]

This PR optimizes swscale in two ways:

Make sure that we allocate the right amount of memory for all planes.

We were not taking sub-sampling into account when allocating planes, resulting in memory allocations that were too big.

Allocate all planes in a single memory region

This makes it possible to e.g. pass a single pointer to APIs that expect planar formats to be a single memory pointer without having to do copies

With liquidsoap, I noticed a serious drop in memory used during video operations:

Before

After

The relevant part is the private memory.