(b1.9-pre2 - 24w35a) Tooltip background

[muzikbike]

Item tooltips have a background graphic that was previously hardcoded. 24w36a made it controlled by editable image files, so here's an opportunity for MC-TextureGen to generate texture files that would generate something visually identical to what existed from b1.9-pre2 to 24w35a.

There are separate "background" and "frame" textures - this ticket tracks the "background".