Make shaders not follow padding; consider moving padding into the renderer

# Description of the new feature/enhancement

I'm not sure whether this is by design or not but making the shader only render inside the padded area makes it, in my opinion, look less good. I like my text to be padded without having my shaders cut off where the text area ends.

For smaller paddings one could currently just move the texture a little by sampling at an offset but this will break highlighting at a certain point.

Start by tracing how shader rendering applies the text area's padding and how highlighting is handled. The work is complete when shaders can render beyond the padded text area without breaking highlighting, while text padding remains unchanged.

Written by the indexing model from the issue text.