New node: Blur (#2477)

* Implementation of gaussian blur and box blur with linear/nonlinear colorspace in raster category

* styling/formatting

* Partial code review

* remove image crate, use conversion functions from color.rs

* fix box blur checkmark, fix linear/gamma conversion

* mult/unmult alpha before/after blur

* Code review

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Co-authored-by: Keavon Chambers <keavon@keavon.com>

node-graph/gstd/src/lib.rs

@@ -8,6 +8,7 @@ pub mod vector;
 pub use graphene_core::*;
 pub mod brush;
 pub mod dehaze;
+pub mod filter;
 pub mod image_color_palette;
 #[cfg(feature = "wasm")]
 pub mod wasm_application_io;