New features: WEBP and text effects

[Cooler2]

Two new features have landed in the engine5 branch.

Text with effects (R-33)

DrawTextFX draws text with glow, outline and shadow. An effect is a stack of layers. Each layer has a color, an offset, a spread (rounded dilation) and a gaussian
blur. Glow, Outline and Shadow constructors cover the common cases:

  DrawTextFX(font,x,y,$FFFFE080,'56',taLeft,
    [TTextEffectLayer.Glow($80000000,8),
     TTextEffectLayer.Outline($FF000000,3)]);

• The text is baked once on the GPU through the regular txt.Write path. Markup, multi-line text, fallback fonts and anchoring work exactly as with plain text.
• The result is cached as a sprite in an LRU cache. Alpha is applied at draw time, so fading text is never re-baked.
• A test checks the GPU result against a CPU reference (within 2/255). Screen 9 of TextDemo shows the effects.

WebP images (R-32)

Static WebP images, lossless and lossy with alpha, now load like PNG/JPEG: LoadImageFromFile, the async preloader and extension probing all accept .webp. Support is
opt-in: define WEBP to link the decode-only libwebp 1.6.0.

Example: an 800×400 logo with a soft alpha glow.

  ┌────────────────────┬────────┐
  │       Format       │  Size  │
  ├────────────────────┼────────┤
  │ PNG                │ 355 KB │
  ├────────────────────┼────────┤
  │ WebP lossless      │ 260 KB │
  ├────────────────────┼────────┤
  │ WebP lossy + alpha │ 120 KB │
  └────────────────────┴────────┘

• Decoder binaries are built from a pinned, hash-checked source release and bundled for Windows x64, Linux x64 and Android arm64. Build scripts are in platform/webp/.
• The Linux CI job runs the decode tests against the bundled library.
• Also fixed: in FPC builds, the PNG fallback reader wrote 4 bytes per pixel into grayscale (Mono8/A8) targets.
• Not included yet: macOS/iOS decoders, animated WebP and AVIF.