feat: add shadow_node

[WillPower3309]

No description provided.

[WillPower3309]

remaining work:

• render around a defined wlr_box to support offset (or maybe we just add the offset back - that may be better actually)
• properly size and position the border node -> we shouldn't need the comp to take blur_sigma into account when defining the shadow width and height. See the scene-graph example for size / position of the shadow node to see what I mean
• update tinywl: draw shadow behind surface
• support transforms
• use node xdg geometry
• Clean up the window box detection logic

[WillPower3309]

My tinywl work is barely functional, will need more work

[WillPower3309]

Updated remaining work comment

[WillPower3309]

We can support multiple "window boxes" as follow-up work. We can use https://www.lighthouse3d.com/tutorials/glsl-tutorial/uniform-variables/ as a handy reference here

[ErikReider]

Everything seems to work as expected, LGTM! :)