gallium: Add a cap to check if the driver supports fill_rectangle

Changes since v1:
- Add pipe caps for etnaviv, freedreno, swr and virgl

Signed-off-by: Lyude <lyude@redhat.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/softpipe/sp_screen.c

@@ -298,6 +298,7 @@ softpipe_get_param(struct pipe_screen *screen, enum pipe_cap param)
    case PIPE_CAP_TGSI_FS_FBFETCH:
    case PIPE_CAP_TGSI_MUL_ZERO_WINS:
    case PIPE_CAP_TGSI_CLOCK:
+   case PIPE_CAP_POLYGON_MODE_FILL_RECTANGLE:
       return 0;
    case PIPE_CAP_SHADER_BUFFER_OFFSET_ALIGNMENT:
       return 4;