fix: word wrapping doesn't work in TUI pager

[leanghok120]

Summary

this pr fixes #878

Problem

running glow -t -w 20 README.md doesn't wrap the words like how it does in the cli version.

Solution

I made it so that when stateShowDocument is set, it updates the m.pager.viewport.width to the value of the -w flag.

After the solution, running glow -t -w 20 README.md wraps the words correctly.

Example

this is the output for glow -t -w 20 README.md:

Before:

After:

Tests

• go test passes

• I have read CONTRIBUTING.md.

• I have created a discussion that was approved by a maintainer (for new features).