mir: set cursor
mir: enable csd
mir: rework cursors a bit

Store the cursor name on the cursor (rather than always using its type).

Use this when setting a cursor on a surface.

The mir server will fallback to using standard cursors from the cursor
theme if the name used is not one of those defined by mir, which is more
or less what we want to happen here in case of creating a cursor by
name.
gdkgl: Make some helper functions static