feat: background-position length type support

[temper357]

Closes #406

• 增加 background-position 对于 length 类型支持，如 background-position: '60px 60px'。
  • W3C 标准中 background-position 的 value 支持三种类型，keyword/percentage/length，其中 keyword 与 percentage 等价都是 background image 在 container 中相对定位，而 length 类型是相对于 container 的 top left 的绝对定位，Kraken 之前是调用 flutter 的 DecorationImage 中的 paint 方法来绘制 image，而 DecorationImage 只支持 alignment 参数，因此之前只支持 keyword/percentage 类型。
  • 本 PR 通过复写 DecorationImagePainter 类，在 paint 方法中传入了 length 类型的 background-position，并且在 canvas 绘制时增加对 length 类型的条件判断来提供对 length 类型的支持。
• 增加 background-position-x 与 background-position-y 样式支持。

[openkraken-bot]

• Total time cost(without paint and layout): 472.174ms ↓ (29%)
• widget_creation_cost: 30.043ms ↓ (30%)
  • controller_properties_init_cost 0.10ms↓ (13%)
  • view_controller_properties_init_cost 0.01ms↓ (33%)
  • element_manager_init_cost 1.50ms↓ (28%)
  • element_manager_property_init 0.01ms↑ (85%)
  • root_element_property_init_cost 0.40ms↓ (37%)
  • root_element_init_cost 0.57ms↓ (27%)
  • create_viewport 0.01ms↓ (72%)
  • bridge_init_cost 28.23ms↓ (30%)
  • bridge_register_dart_method_cost 12.07ms↓ (26%)
  • js_context_init_cost 5.16ms↓ (37%)
  • native_method_init_cost 3.06ms↓ (34%)
  • polyfill_init_cost 7.45ms↓ (30%)
• First Bundle Load: 442.13100000000003ms ↓ (28%)
  • js_bundle_load_cost 107.90ms↓ (29%)
  • js_bundle_eval_cost 281.84ms↓ (28%)
  • js_parse_time_cost 29.46ms↓ (29%)
  • flush_ui_command_cost 11.39ms↓ (39%) avg: 0.57ms↓ (39%) count: 2.00 (0%)
  • create_element_cost 2.72ms↑ (28%) avg: 0.01ms↑ (30%) count: 19.50 (0%)
  • js_host_class_get_property_cost 53.12ms↓ (23%) avg: 0.01ms↓ (28%) count: 573.40 (0%)
  • js_host_class_set_property_cost 37.29ms↓ (28%) avg: 0.01ms↓ (28%) count: 273.50 (0%)
  • js_host_class_init_cost 0.00ms(0%)
  • js_native_function_call_cost 35.83ms↓ (29%) avg: 0.02ms↓ (28%) count: 184.30 (0%)
  • create_text_node_cost 0.22ms↓ (37%) avg: 0.01ms↓ (28%) count: 4.70 (0%)
  • create_comment_cost 0.14ms↓ (23%) count: 11.60 (0%)
  • dispose_event_target_cost 0.00ms(0%)
  • add_event_cost 0.52ms↓ (26%) count: 34.10 (0%)
  • insert_adjacent_node_cost 21.93ms↓ (24%) avg: 0.06ms↓ (22%) count: 35.60 (0%)
  • remove_node_cost 0.01ms↓ (28%) count: 0.50 (0%)
  • set_style_cost 9.75ms↓ (28%) avg: 0.01ms↓ (28%) count: 205.40 (0%)
  • dom_force_layout_cost 48.68ms↓ (27%) avg: 9.74ms↓ (27%) count: 0.50 (0%)
  • dom_flush_ui_command_cost 54.26ms↓ (28%) avg: 0.38ms↓ (28%) count: 14.30 (0%)
  • set_properties_cost 5.71ms↓ (44%) avg: 0.01ms↓ (37%) count: 86.00 (0%)
  • remove_properties_cost 0.00ms(0%)
• Rendering: 53.793000000000006ms ↓ (25%)
  • flex_layout_cost 21.13ms↓ (25%) avg: 0.08ms↓ (23%) count: 24.70 ↓ (9%)
  • flow_layout_cost 8.39ms↓ (8%) avg: 0.08ms↓ (8%) count: 9.50 ↓ (8%)
  • intrinsic_layout_cost 0.83ms↓ (12%) avg: 0.01ms↓ (28%) count: 12.40 (0%)
  • silver_layout_cost 0.00ms(0%)
  • paint_cost 23.45ms↓ (31%) avg: 0.01ms↓ (35%) count: 151.80 ↓ (21%)