Fix crt and vhs shader brightness.

[cookiemonsternz]

Just changed the brightness value, makes it look slightly worse on GitHub dark, but much better on all other themes. Might be worth trying different brightness values and adjusting the pixel sizes for the shader, as when it is not overwhelmed with brightness it does look a little worse. Over around brightness 1.5 makes line numbers unreadable for GitHub light theme, however.

Github Dark Before:

Github Dark After:

One Dark Pro Darker Before:

One Dark Pro Darker After:

Github Light Before:

Github Light After:

Nord Before:

Nord After:

[TheMasterCoders]

Sacrifice of the GitHub Dark is required to let the rest get fixed if you choose to merge this.