Keep the glyph uvs as vectors on both sides

Review response: `GlyphInfo` goes back to two `vec2<f32>`, which is what
the uvs are, and `GlyphPrimitive` takes `#[repr(C, align(8))]` to match.
That leaves four padding bytes, which the `primitives!` macro's
`unsafe impl Pod` accepts. Measured at 32 bytes, align 8, the same as
WGSL's layout for the struct.

core/src/render/primitive.rs

@@ -89,6 +89,8 @@ macro_rules! primitives {
         }
 
         $(
+            // Each is uploaded as its WGSL counterpart, so its fields have to
+            // sit at WGSL's offsets -- including an align Rust would not pick.
             unsafe impl bytemuck::Pod for $ty {}
             unsafe impl bytemuck::Zeroable for $ty {}
             impl Primitive for $ty {
@@ -263,7 +265,10 @@ impl RectPrimitive {
 
 /// `color` is multiplied by the atlas alpha for a mask glyph; a colour glyph
 /// takes the texel unchanged, which `GlyphEntry::IS_COLORED` selects.
-#[repr(C)]
+///
+/// `align(8)` because `vec2<f32>` aligns `GlyphInfo` to 8, which pads it to
+/// 32 bytes.
+#[repr(C, align(8))]
 #[derive(Debug, Copy, Clone)]
 pub struct GlyphPrimitive {
     pub uv_min: Vec2,

core/src/render/shader.wgsl

@@ -17,13 +17,9 @@ struct Rect {
     inner_radius: f32,
 }
 
-// Scalars rather than two vec2<f32>: a vec2 member would align the struct to
-// 8 and pad it to 32 bytes, which GlyphPrimitive has no field to fill.
 struct GlyphInfo {
-    uv_min_x: f32,
+    uv_min: vec2<f32>,
-    uv_min_y: f32,
+    uv_max: vec2<f32>,
-    uv_max_x: f32,
-    uv_max_y: f32,
     layer: u32,
     color: u32,
     flags: u32,
@@ -162,9 +158,7 @@ fn draw_texture(region: Region) -> vec4<f32> {
 }
 
 fn draw_glyph(region: Region, g: GlyphInfo) -> vec4<f32> {
-    let uv_min = vec2(g.uv_min_x, g.uv_min_y);
+    let uv = mix(g.uv_min, g.uv_max, region.uv);
-    let uv_max = vec2(g.uv_max_x, g.uv_max_y);
-    let uv = mix(uv_min, uv_max, region.uv);
     let texel = textureSample(tex, samp, uv, i32(g.layer));
     if (g.flags & 1u) != 0u {
         return texel;