So @stof and @romainneutron, what do you guys reckon?

I've been looking a bit more into GraphicsMagick and they maintain much the same policy as ImageMagick. They don't automatically resize all layers when you resize the image on a whole because of the different semantics.

In a PSD, different layer sizes are absolutely intentional, so resizing the image as a whole would involve resizing each layer proportional.

For an animated GIF however, frames with different sizes are merely a result from optimising the GIF; proportional resizing won't work, because the optimised frame is not necessarily a portion of the main image. Even the colour table may differ per frame.

That's what coalescing fixes; it more or less "de-optimises" the frames so that they match the first frame's resolution, colour table and so on to become a full frame again.

Now comes the really iffy part: `ext-gmagick` didn't add support for -coalesce until 1.1.2RC1 ([Changelog on PECL](http://pecl.php.net/package-changelog.php?package=gmagick)). That puts Imagine in a really awkward position. The later versions break Imagine on a whole due to some incompatibility in the point format. Moreover, before 1.1.2RC1 there's some automatic coalescing going on in the extension that is *not* part of GraphicsMagick itself. I think I'm going to dig deeper into that, because right now Imagine isn't usable with newer versions of ext-gmagick.

For this particular pull request and the issue at hand here, it also means that for older and newer versions there'd have to be a version distinction to use `coalesceImage()` or not. This is best served with a `supportCoalesce()` method that checks that internally regardless of how you want to move forward with the overall `resize()` method, so I'll add that.