genxml: Make a SCISSOR_RECT structure on Gen4-5.

genxml: Make a SCISSOR_RECT structure on Gen4-5.

Gen6+ support multiple scissor rectangles, and define a SCISSOR_RECT
structure containing their dimensions.  On Gen4-5, those same fields
exist in SF_VIEWPORT.

This patch extracts the SF_VIEWPORT fields into a SCISSOR_RECT
structure.  Although not a named concept on Gen4-5, it works just
as well, and gives us a consistent SCISSOR_RECT structure across
all generations, making it easier to reuse code.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>