native_render: HiDPI glyph pages, sharp UI upscaling, 4x MSAA

- CGraphicFontTexture rasterises glyph pages at the swapchain density
  (UISetFontOversample, MT_FONT_OVERSAMPLE) with anti-aliased coverage;
  metrics, packing and UVs stay the 40250 1x values. Default 1 keeps the
  verbatim bilevel path (Godot unchanged).
- native.frag: sharp-bilinear sampling for magnified UI textures.
- Multisampled colour/depth with a resolve into the swapchain (MT_MSAA,
  default 4); stats line reports msaa=N.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

native_render/native.frag

@@ -105,7 +105,20 @@ void main() {
         return;
     }
     // 2D UI batch (UIRenderCommand): vertex color times texture, optionally clipped by a mask.
-    vec4 color = in_color * texture(tex_sampler, in_uv);
+    // The UI is laid out in logical pixels and magnified onto a HiDPI swapchain. Plain bilinear
+    // smears each texel over the scale factor; "sharp bilinear" keeps texels square and only blends
+    // across a one-pixel seam. Minified or 1:1 texels keep the original bilinear lookup.
+    vec2 uv = in_uv;
+    vec2 tex_size = vec2(textureSize(tex_sampler, 0));
+    vec2 texel = uv * tex_size;
+    vec2 texels_per_pixel = fwidth(texel);
+    if (texels_per_pixel.x > 0.0 && texels_per_pixel.y > 0.0 &&
+        texels_per_pixel.x < 0.99 && texels_per_pixel.y < 0.99) {
+        vec2 seam = floor(texel + 0.5);
+        texel = seam + clamp((texel - seam) / texels_per_pixel, -0.5, 0.5);
+        uv = texel / tex_size;
+    }
+    vec4 color = in_color * texture(tex_sampler, uv);
     if (draw.params.z > 0.5) {
         if (in_mask_uv.x < 0.0 || in_mask_uv.x > 1.0 || in_mask_uv.y < 0.0 || in_mask_uv.y > 1.0)
             discard;