asahi: use GS for first-provoking fans

workaround hw bug / missing feature, in a vk compliant way.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26614>

src/gallium/drivers/asahi/agx_state.c

@@ -3621,6 +3621,24 @@ agx_needs_passthrough_gs(struct agx_context *ctx,
       return true;
    }
 
+   /* Experimentally, G13 does not seem to pick the right provoking vertex for
+    * triangle fans with first provoking. Inserting a GS for this case lets us
+    * use our (correct) shader-based input assembly, translating to
+    * appropriately oriented triangles and working around the hardware issue.
+    * This warrants more investigation in case we're just misconfiguring the
+    * hardware, but as tri fans are absent in Metal and GL defaults to last
+    * vertex, this is a plausible part of the hardware to be broken (or absent).
+    *
+    * Affects piglit clipflat.
+    */
+   if (info->mode == MESA_PRIM_TRIANGLE_FAN &&
+       ctx->rast->base.flatshade_first &&
+       ctx->stage[MESA_SHADER_FRAGMENT].shader->info.inputs_flat_shaded) {
+
+      perf_debug_ctx(ctx, "Using passthrough GS due to tri fan bug");
+      return true;
+   }
+
    /* Transform feedback is layered on geometry shaders, so if transform
     * feedback is used, we need a GS.
     */