Implement support for XR_KHR_composition_layer_equirect (equirect1).

This series of patches prepares the code base for having both equirectangular compositor layer extensions implemented. In general all equirect2 functionality was suffixed with a 2, where equirect1 functionality was suffixed with a 1 to preserve consistency and align more with the spec. Before this patch set equirect2 was mostly called equirect without a suffix. Also equirect1 had either a "legacy" or no suffix.

For testing the extension a xrgears branch using equirec1 instead of 2 can be used: https://gitlab.freedesktop.org/monado/demos/xrgears/-/commits/equirect1

In addition, the compositor related code was made optional at compile time to respect the cmake and meson configuration variables.