gltrace: Send vertex attribute data after glDraw() call.

This patch enables tracing of vertex attribute data that
is specified using glVertexAttribPointer().

At the time the glVertexAttribPointer() call is made, we
only receive a pointer in client space, without any indication
of the size (# of attributes). This size is known only at
the time of the glDraw() call.

This patch generates a new message glVertexAttribPointerData()
when a draw call is issued that contains the vertex attribute
data.

A glDrawArrays() call directly gives the size of data to copy.
A glDrawElements() call gives the indices to copy. In such a
case, all data between the min & max indices drawn are copied
and sent to the host. To support glDrawElements() with an
element array buffer, this patch also adds state that maintains
a copy of all element array buffers.

Change-Id: I434da794a0aa9ada8e7474e219ffb1d79b183ecf