Image texel write: allow writing scalar values into scalar images

[Firestar99]

• Breaking: Image::write() no longer takes any vector component count (eg. for float images Vec2, Vec3 or Vec4), now type must match image's SampleResult exactly
• usage of this function is relatively rare, since it's only used in compute shader (not fragment!) write directly to images, which more advanced graphics pipelines often do for post-processing effects
• Image!(type=f32) aka. not specifying a specific image format (generic format=Unknown): SampleResult=Vec4 instead of allowing any of Vec2, Vec3 or Vec4 (loads similarly always return a Vec4)
• for Image!(format=rg32f): rg -> Vec2, rgb -> Vec3, rgba -> Vec4
• for Image!(format=r32f): broken previously since you couldn't pass in a f32 as it doesn't impl Vector, now allows passing f32 as expected

close #412

[LegNeato]

I don't know enough about this area to review, @schell or @eddyb ?