Dark text colors fix (#15794)

# Objective

Fix the text colors.

fixes #15788

## Solution

Add the missing corner flags.

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(),