gallium: add a rasterizer state bit for unrestricted depth values.

This tells the drivers to expect depth values outside the 0..1 range.

This is used for support VK_EXT_depth_range_unrestricted, but could
also be used for GL_NV_depth_buffer_float in GL.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17612>

src/gallium/include/pipe/p_state.h

@@ -184,6 +184,14 @@ struct pipe_rasterizer_state
     */
    unsigned offset_units_unscaled:1;
 
+   /**
+    * Depth values output from fragment shader may be outside 0..1.
+    * These have to be clamped for use with UNORM buffers.
+    * Vulkan can allow this with an extension,
+    * GL could with NV_depth_buffer_float, but GLES doesn't.
+    */
+   unsigned unclamped_fragment_depth_values:1;
+
    /**
     * Enable bits for clipping half-spaces.
     * This applies to both user clip planes and shader clip distances.