This adds a lovely patch that allows for shaders to be applied any UI element that is drawn using UIElement:draw_pixellated_rect().

In order to apply a shader to a UI element, you must add a new config parameter, shader, to the UI node you want to apply a shader to. This may be structured in a few ways:

• shader = "shader_key", which will simply use the shader SMODS.Shaders["shader_key"] with default arguments being sent.
• shader = { shader = "shader_key", send = tbl}, which uses SMODS.Shaders["shader_key"], while sending arguments defined by tbl (this works the same way as the _send argument in Sprite:draw_shader())
• A table of tables formatted either of the above two ways, which will result in each one being drawn in order, akin to how Sprite:define_draw_steps() functions. Using the string "none" or "dissolve" as the shader key will instead result in no shader being used, which is useful for layered effects.

UI element shaders, by default, use a different parameter set than the typical sprite shaders, so they must be custom-made to work with this. The only arguments shared between them are screen_scale, time (which uses a placeholder value), and the one using the shader's key. UI element shaders additionally send both the position and size of the UI element as uibox_pos and uibox_size, respectively.

To see an example of this in action, Entropy currently has an implementation of UI element shaders I made earlier (which I modified to create this PR), used to apply a shader to its mod badge. Arrow API also has a separate implementation to apply the Polychrome shader to UI elements.

I may come back to this to add support for text nodes and DynaText objects, but I'm already very happy with this.

Additional Info:

• I didn't modify api's or I've made a PR to the wiki repo.
• I didn't modify api's or I've updated lsp definitions.
• I didn't make new lovely files or all new lovely files have appropriate priority.