Disallow flipped source coordinates in Surface.drawImage()

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The sprite-rendering code in https://github.com/joelgwebber/xna-platformer 
takes advantage of flipping source coordinates in Surface.drawImage() to flip 
sprites horizontally. This works in Java2D and GL, but fails in <canvas>, where 
it's explicitly disallowed.

We should assert that source coordinates are in the correct order so that all 
platforms will be consistent. We also need a way to render flipped sprites, 
though the easiest thing to do there might simply be to invert the scale (see 
issue 32 for more on that).

Original issue reported on code.google.com by joelgwebber on 7 Jun 2011 at 1:29

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Original comment by [email protected] on 7 Jun 2011 at 1:33

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