See RT-32242 which covers the fillRect and clearRect cases. Images without an alpha channel, or images with an alpha that are painted with the appropriate compositing mode should also flush the Canvas command buffer when appropriate.

Since image data can be large it is likely to fill the command buffer quickly. Just a momentary stall in painting could result in massive memory usage. RT-24903 is filed against that case, but this issue would reduce the frequency of the problem until that is addressed as well as being a performance improvement by skipping the unnecessary commands.