Bug 896129. Use CGContextDrawImage when appropriate in FillRect(). r=mattwoodrow

This fixes the element-paint-paintserversize-rounding-02.html reftest.
Bug 893029 - Consider making the tab 'plus' button a larger target

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extra : rebase_source : b065cdc5f81dd54a973fa303e8bea2c1e06d605f