i965: Force outputs_written to contain varyings needed by stream-out.

i965: Force outputs_written to contain varyings needed by stream-out.

If transform feedback is recording a varying, it needs a slot in the
VUE map, regardless of whether or not the shader writes it.

Together with the previous patch, this fixes:
- KHR-GL45.enhanced_layouts.xfb_capture_struct

The test captures a structure where the vertex shader writes the first
and third members - but the second still needs a slot.

Reviewed-by: Juan A. Suarez Romero <jasuarez@igalia.com>