GLX: fix context render type queries

Querying the GLX_RENDER_TYPE of a GLX context via glXQueryContext will
currently return the render type of the context's FB config, which is
a bitmask of GLX_RGBA_BIT / GLX_COLOR_INDEX_BIT / ... values. However,
this query should really return the render type that was specified
when creating the context, which is one of GLX_RGBA_TYPE /
GLX_COLOR_INDEX_TYPE / .... To enable this, save the render type when
creating a new context (defaulting to GLX_RGBA_TYPE if unspecified),
and then include this value in the context attributes sent to clients.
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