Bug 1028786, part 1 - Add a gfxUtils::ClearThebesSurface() helper method. r=mattwoodrow

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extra : rebase_source : 524ffe6d276ea9b51cf01d534f65c9b74b32b950
Bug 1028786, part 2 - Get rid of the Thebes backed gfxContext in gfxPlatformGtk.cpp. r=mattwoodrow

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extra : rebase_source : 2920a2b3d862180a15d5a037741744ea5766b5b1