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DrawManager: Engine Instance Data.

In the original design draw engines had to copy with a limitation that
they were not allowed to reuse complex data structures between drawing
calls. Data that could be reused were limited to:
- GPUFramebuffers
- GPUTextures
- Memory that could be removed calling MEM_freeN (storage list)
- DRWPass

This is fine when the storage list contains arrays or structs but when
more complex data types (vectors, maps) etc wasn't possible.

This patch adds instance_data that can be reused between drawing calls.
The instance_data is controlled by the draw engine and doesn't need to
be limited as described above.

When an engines stores instance_data it must implement the
`DrawEngineType.instance_free` callback to free the data.

The patch originates from eevee rewrite. But was added to master as the
image engine rewrite also has a need for it.

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D13425