Niels De Graef authored Oct 14, 2019 and Jonas Ådahl committed Oct 15, 2019

As we will start adding support for more pixel formats, we will need to
define a notion of planes. This commit doesn't make any functional
change, but starts adding the idea of pixel formats and how they (at
this point only theoretically) can have multple planes.

Since a lot of code in Mutter assumes we only get to deal with single
plane pixel formats, this commit also adds assertions and if-checks to
make sure we don't accidentally try something that doesn't make sense.

!858