Fix text shadow outline draw batching.

[bruvzg]

Fixes #103464

Drawing shadow outline for all glyphs first, instead of drawing both shadow and shadow outline for each glyph, seems to be almost 2x faster. Also removes some unnecessary shadow related processing from RTL.

[clayjohn]

Makes sense. The shadow outline must be breaking batching (does it use a different texture?), reordering the draws allows Godot to render this in 2 draw calls and saves a ton of resources.

[bruvzg]

does it use a different texture?

Yes, outline is always in a different texture.

[Calinou]

Code looks good to me.