Improve text sharpness (#5838)

This improves the sharpness of text slightly, thanks to
<https://hikogui.org/2022/10/24/the-trouble-with-anti-aliasing.html> by
@hikogui (🙏 ).

The difference is very small, but in dark mode (bright text on dark
background) text is sometimes significantly sharper, and also slightly
brighter.

The difference in light mode (dark text on light background) is much
smaller (barely perceptable).

To compare the before/after I suggest you open both in new tabs, then
quickly change between them.

### Low-DPI
#### Before
![Code Editor
old](https://github.com/user-attachments/assets/e10a3cad-932f-48cd-b7d6-5bfe70954c5e)

#### After
![Code
Editor](https://github.com/user-attachments/assets/2e7383fe-8023-4425-91c8-93df3c22c0fe)

#### Before

![old-white](https://github.com/user-attachments/assets/51c41c59-e897-4831-857a-f3ffe17ce7d4)

#### After

![new-white](https://github.com/user-attachments/assets/4ac6f951-8c57-4bcc-a5d5-788cf52ea7d8)

### High-DPI
The difference is starkest on low-DPI screens (like the above
screenshots). On high-DPI screens, the difference is much smaller

#### Before
![widget_gallery
old](https://github.com/user-attachments/assets/f2149a30-aef8-4383-b48c-73d33a03ca02)

#### After

![widget_gallery](https://github.com/user-attachments/assets/c9ceb8be-8a32-490c-9364-2c6562b741f6)

crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.00.png

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crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.25.png

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crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_2.00.png

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