`ImageFilter.compose`: sampling outside of inner has backend differences

The differences have to do with taking away the extra 1 pixel padding from the RenderToSnapshot introduced in https://github.com/flutter/flutter/pull/177687. The differences show up in `ComposePaintRuntimeInner` and `ComposePaintRuntimeOuter` tests. The output I would expect is actually opengles in this case, the vulkan and metal outputs look wrong.

This removal of the padding was required to get pixel perfect fragment shading coordinates until we can potentially pass a transform into the ImageFilter.shader shader to represent the offset.

## ComposePaintRuntimeInner

### vulkan

### opengles

### metal