This way the grabbing side can indicate when the clipboard owning app
can provide the clipboard contents in multiple formats, ie plain text
and html.

Also drop the VD_AGENT_CLIPBOARD_BITMAP type until we have a clear
cross platform definition of the contents to send for bitmap type
clipboard data.

VD_AGENT_CLIPBOARD_NONE for unsupported clipboard types

-VD_AGENT_CLIPBOARD_GRAB(type) - tell the other side that an application in our side ("we") got ownership of the clipboard.
-VD_AGENT_CLIPBOARD_REQUEST(type) - after we know the other side owns the clipboard (GRAB received), we notify the os we are the owner. when we are asked by the os/app for the clipboard data, we send this REQUEST msg to the other side.
-VD_AGENT_CLIPBOARD(type, data) - the existing message for sending the clipboard, is now sent only in response to REQUEST.
-VD_AGENT_CLIPBOARD_RELEASE - tell the other side that we are no longer the owner of the clipboard (e.g. the owner app was closed).

this patch will be followed by agent & client patches handling the above messages.

spice 0.6 uses 32bpp values unconditionally for brush and palette
colors.

spice 0.4 used to use 16 bpp or 32 bpp depending on the video mode.
The qxl parser needs to know the guest video mode depth to correctly
interpret these values in spice 0.4 compat mode.  Add a flag to pass
on this informartion.