update to build with cogl master

cogl_offscreen_new_to_texture was recently renamed to
cogl_offscreen_new_with_texture so this updates Rig to use the new name
so it builds with cogl master.
rig: add basic copy and paste support

This adds initial copy and paste support. RutShell now keeps track of a
current selection that must implement a new 'selectable' interface and
while there is something selected will handle ctrl-c/x key bindings to
be able to copy/cut the selection into the clipboard and from there
allow copying and pasting via ctrl-v. The actual pasting is implemented
with a 'mimable' interface and a new _DROP input event and the hope is
that we can also build on these for drag and drop support later.

Note: this doesn't currently integrate with the operating system's
native copy & paste apis so you can only copy and paste within Rig
itself.
Enable  select/copy/paste for asset inspectors

This makes asset inspectors selectable and adds visual feedback when an
asset inspector is selected. This also make assets mimable so they can
be copy and pasted.

Note: this change highlights a limitation with how renderers can track
changes to components. Currently we rely on rig-engine.c to notify the
renderer when we make changes to the image source of a material
component, but an asset inspector is not currently able to notify the
renderer and so updates via an asset inspector aren't reflected in
the UI preview. This requires some re-working of the renderer to fix
properly.
entity: Add renderer_priv member

This adds a general way for renderers to track their own private state
associated with entities so we don't have to maintain lots of fixed size
cache arrays.