Create class for managing image buffer pools

[jtackaberry]

Some widgets, like rtk.Entry and rtk.Viewport, create their own backing stores. As Reaper has a fairly low limit on the number of available image buffers (1024), this restricts the number of those widgets that can be created.

The idea is to create a class that works along side rtk.Image to create rtk-internal buffers. REAPER images can be 8192x8192 in size, and with 1024 of them, that's quite a lot of image buffer space to work with. We just need to be cleverer about allocation.

The buffer pool class should:

• dynamically allocate REAPER buffers (using rtk.Image) with the full 8192x8192 resolution as needed
• provide an API to allocate rtk buffers (sub regions from the REAPER buffer), bin-packing across available REAPER buffers based on the requested geometry. The subregion would use rtk.Image:viewport()
• maintain a refcount for each REAPER buffer
• support Lua GC to automatically release subregions and decref the REAPER buffer
• free REAPER buffers when no more refs exist