Invert classification crashes on screen resize

[lilleyse]

Steps to reproduce:

• Run Classification sandcastle demo
• Check "invert classification"
• Change alpha to something < 1.0
• Resize window

I also noticed that when invert classification is enabled with alpha set to 1.0 each pick call causes the framebuffers in InvertClassification to be recreated. Since the draw pass sets previousFramebuffer to the globe or FXAA framebuffer and the subsequent pick sets previousFramebuffer to undefined, the dirty flag is constantly getting set.

Reported on the forum: https://groups.google.com/forum/#!topic/cesium-dev/1UTvmlnyzzs

[pjcozzi]

@bagnell can you please look at this soon so it makes the next release on Jan 1?

[cesium-concierge]

Congratulations on closing the issue! I found these Cesium forum links in the comments above:

https://groups.google.com/forum/#!topic/cesium-dev/1UTvmlnyzzs

If this issue affects any of these threads, please post a comment like the following:

The issue at #6046 has just been closed and may resolve your issue. Look for the change in the next stable release of Cesium or get it now in the master branch on GitHub https://github.com/AnalyticalGraphicsInc/cesium.

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