Move the Gaussian blur shader function into a reusable library.

[pcwalton]

The depth of field pass has a medium-performance Gaussian blur implementation that's well-commented and battle-tested, but it's currently private to that pass. Gaussian blur is useful enough that it's worth exposing to application shaders, so this PR factors that code out into a reusable shader library.

[ChristopherBiscardi]

single-directional blur works when dropped into a post-process, etc.

One note is that the module naming is a bit unfortunate, as importing bevy_post_process::gaussian_blur instead of bevy_post_process::gaussian_blur::gaussian_blur will result in silent failures when trying to actually use the function.

[hukasu]

does this work on both bindless (if applicable) and non-bindless? because i tried doing the same with one other shader function and wgpu crashes when passing a bindless texture_2d to it

see #21714

[ChristopherBiscardi]

@hukasu No, this crashes on my windows machine in the shader_material_bindless example, but since its just refactoring that shouldn't block the PR IMO