Dark text colors fix (#15794)

# Objective

Fix the text colors.

fixes #15788

## Solution

Add the missing corner flags.

Author: ickshonpe

crates/bevy_ui/src/render/mod.rs

@@ -1176,7 +1176,7 @@ pub fn prepare_uinodes(
                                         position: positions_clipped[i].into(),
                                         uv: uvs[i].into(),
                                         color,
-                                        flags: shader_flags::TEXTURED,
+                                        flags: shader_flags::TEXTURED | shader_flags::CORNERS[i],
                                         radius: [0.0; 4],
                                         border: [0.0; 4],
                                         size: size.into(),