Add gradient text effect

[waldnercharles]

Text rendering:

• Added per-corner color support to CF_TextEffect via a colors[4] array and use_colors flag, enabling individual colors for each glyph corner.
• Implemented the <gradient> text effect, allowing for horizontal, vertical, and per-corner gradients via markup (e.g., <gradient left=#ff0000 right=#0055ff>...</gradient>). Includes logic to blend edge and corner colors and applies them per-glyph.
• Updated the text rendering pipeline to use per-corner colors when the effect is active, falling back to flat color otherwise.
• Modified the rendering batch system to pass per-corner colors for text glyphs to the GPU.

Sample:

• Expanded the text drawing sample to showcase the new gradient and per-corner color features, with several example strings demonstrating different gradient configurations. Also increased the window size.

Other:

• Fixed a bug in s_parse_color where it would attempt to parse the next field if it starts with valid hex characters.

[chatgpt-codex-connector]

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