Prep gcore splitup: move various symbols into their own modules (#2746)

* move `trait AsU32` from `gcore::vector::misc` to `gcore`

* move blending and gradient to their own modules

* fix unused warnings

* move `Quad`, `Rect` and `BBox` to `gcore::math`

* extract `ReferencePoint` and transform nodes from `transform`

* move color-related code to `mod color`

* fix unused warning in test code

* move blending-related nodes and code to `mod blending_nodes`

* move ClickTarget code to `mod vector::click_target`

node-graph/gcore/src/math/mod.rs

@@ -0,0 +1,4 @@
+pub mod bbox;
+pub mod math_ext;
+pub mod quad;
+pub mod rect;