vrend: Read only as many bytes as were mapped

The width given in glMapBufferRange refers to machine units, i.e. the format of the underlying buffer is of no consequence. Hence only info->box->width bytes are mapped and can be copied to the IOV.

In guest mesa this is accounted for so we don't have to calculate the send_size value.

Closes: https://gitlab.freedesktop.org/virgl/virglrenderer/-/issues/469

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