Fix vertically-flipped content in the GLES path-clip composite

The clip-layer composite shader sampled the offscreen layer at the wrong vertical position, so path-clipped content (e.g. a circular avatar) came out upside down on the GLES backend.
`v_uv.y` runs bottom-to-top in screen space — the layer FBO has GL's lower-left origin, and `ortho_rect` plus the texture shader's flip establish that `v_uv.y == 1` is the top edge. The composite computed the fragment's screen Y as `bbox.y + v_uv.y * bbox.h`, which is the inverted Y, so it read the layer mirrored about the horizontal axis. Compute it as `bbox.y + (1 - v_uv.y) * bbox.h` instead. The mask sampling was already correct (and a symmetric circle hid the flip in the example; a photo reveals it). The software backend is unaffected — it clips through a coverage mask with no layer round-trip.

src/gles_render/shaders.rs

@@ -163,7 +163,10 @@ uniform vec2 u_canvas;
 uniform vec4 u_bbox;
 void main()
 {
-	vec2 sp  = u_bbox.xy + v_uv * u_bbox.zw;
+	// `v_uv.y` runs bottom-to-top in screen space (FBO origin is lower-left, as
+	// `ortho_rect` + the texture shader's flip establish), so the screen Y of
+	// this fragment is `bbox.y + (1 - v_uv.y) * bbox.h`.
+	vec2 sp  = vec2( u_bbox.x + v_uv.x * u_bbox.z, u_bbox.y + ( 1.0 - v_uv.y ) * u_bbox.w );
 	vec2 luv = vec2( sp.x / u_canvas.x, 1.0 - sp.y / u_canvas.y );
 	vec4 col = texture2D( u_layer, luv );
 	float cov = texture2D( u_mask, vec2( v_uv.x, 1.0 - v_uv.y ) ).a;