draw: don't crash on vertex buffer overflow

We would crash when stride was bigger than the size of the buffer.
The correct behavior is to just fetch zero's in this case.
Unfortunatly with user_buffer's there's no way to validate the size
because currently we're just not getting it. Adjust the draw interface
to pass the size along the mapped buffer, which works perfectly
for buffer backed vertex_buffers and, in future, it will allow
us to plumb user_buffer sizes through the same interface.

Signed-off-by: Zack Rusin <[email protected]>
Reviewed-by: José Fonseca <[email protected]>
Reviewed-by: Roland Scheidegger <[email protected]>

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