gallium: add PIPE_CAP_QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION

Just let the hardware do it if it can and avoid drivers having to
check for the special case on each draw call.

v2: update the draw module

src/gallium/docs/source/cso/rasterizer.rst

@@ -59,13 +59,14 @@ flatshade_first
 Whether the first vertex should be the provoking vertex, for most primitives.
 If not set, the last vertex is the provoking vertex.
 
-There are several important exceptions to the specification of this rule.
+There are a few important exceptions to the specification of this rule.
 
 * ``PIPE_PRIMITIVE_POLYGON``: The provoking vertex is always the first
   vertex. If the caller wishes to change the provoking vertex, they merely
   need to rotate the vertices themselves.
-* ``PIPE_PRIMITIVE_QUAD``, ``PIPE_PRIMITIVE_QUAD_STRIP``: This option has no
-  effect; the provoking vertex is always the last vertex.
+* ``PIPE_PRIMITIVE_QUAD``, ``PIPE_PRIMITIVE_QUAD_STRIP``: The option only has
+  an effect if ``PIPE_CAP_QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION`` is true.
+  If it is not, the provoking vertex is always the last vertex.
 * ``PIPE_PRIMITIVE_TRIANGLE_FAN``: When set, the provoking vertex is the
   second vertex, not the first. This permits each segment of the fan to have
   a different color.

src/gallium/docs/source/screen.rst

@@ -96,6 +96,8 @@ The integer capabilities:
   controlled through pipe_rasterizer_state.
 * ``PIPE_CAP_GLSL_FEATURE_LEVEL``: Whether the driver supports features
   equivalent to a specific GLSL version. E.g. for GLSL 1.3, report 130.
+* ``PIPE_CAP_QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION``: Whether quads adhere to
+  the flatshade_first setting in ``pipe_rasterizer_state``.