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A plane is one of three possible: primary, overlay and cursor. Each plane can have various properties, such as possible rotations, formats etc. Each plane can also be used with a set of CRTCs. A primary plane is the "backdrop" of a CRTC, i.e. the primary output for the composited frame that covers the whole CRTC. In general, mutter composites to a stage view frame onto a framebuffer that is then put on the primary plane. An overlay plane is a rectangular area that can be displayed on top of the primary plane. Eventually it will be used to place non-fullscreen surfaces, potentially avoiding stage redraws. A cursor plane is a plane placed on top of all the other planes, usually used to put the mouse cursor sprite. Initially, we only fetch the rotation properties, and we so far blacklist all rotations except ones that ends up with the same dimensions as with no rotations. This is because non-180° rotations doesn't work yet due to incorrect buffer modifiers. To make it possible to use non-180° rotations, changes necessary include among other things finding compatible modifiers using atomic modesetting. Until then, simply blacklist the ones we know doesn't work. GNOME/mutter#548 GNOME/mutter!525
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