app: don't chunk update area when rendering projection synchronously
Add a boolean "chunk" parameter to gimp_projection_chunk_render_iteration(), which determines whether the work area should be sub-divided into chunks prior to rendering (previously, the work area would always be sub-divided.) Only pass TRUE when rendering the projection asynchronously, in the render callback, and pass FALSE when rendering the projection synchronously, in gimp_projection_finish_draw(), which is called when flushing the projection through the GimpPickable interface. Rendering the projection using as big chunks as possible improves performance, while worsening responsiveness. Since responsiveness doesn't matter when rendering synchronously, there's no reason to render in chunks.
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