Harden runtime flags and thin Apple/canvas seams

src/legacy_canvas_draw_merge_services.h

@@ -2,6 +2,7 @@
 
 #include "shader.h"
 
+#include <array>
 #include <functional>
 
 namespace pp::panopainter {
@@ -238,6 +239,14 @@ struct LegacyCanvasDrawMergePostDrawExecution {
     std::function<void()> draw_current_modes;
 };
 
+struct LegacyCanvasDrawMergeSmaskFacesExecution {
+    std::function<void()> set_active_texture_unit;
+    std::function<void()> enable_blend;
+    std::function<void(int plane_index)> bind_face_texture;
+    std::function<void()> draw_face;
+    std::function<void(int plane_index)> unbind_face_texture;
+};
+
 [[nodiscard]] inline LegacyCanvasDrawMergeShaderExecution legacy_shader_manager_draw_merge_execution() noexcept
 {
     return {
@@ -631,4 +640,29 @@ inline void execute_legacy_canvas_draw_merge_post_draw(
     execution.draw_current_modes();
 }
 
+inline void execute_legacy_canvas_draw_merge_smask_faces(
+    const LegacyCanvasDrawMergeTextureMaskUniforms& uniforms,
+    const glm::mat4& proj,
+    const glm::mat4& camera,
+    float layer_scale,
+    const std::array<glm::mat4, 6>& plane_transform,
+    const LegacyCanvasDrawMergeSmaskFacesExecution& execution)
+{
+    setup_legacy_canvas_draw_merge_texture_mask_shader(uniforms);
+    execution.set_active_texture_unit();
+    execution.enable_blend();
+
+    for (int plane_index = 0; plane_index < 6; ++plane_index) {
+        auto plane_mvp = proj * camera *
+            glm::scale(glm::vec3(layer_scale)) *
+            plane_transform[plane_index] *
+            glm::translate(glm::vec3(0, 0, -1.f));
+
+        apply_legacy_canvas_draw_merge_mvp(plane_mvp);
+        execution.bind_face_texture(plane_index);
+        execution.draw_face();
+        execution.unbind_face_texture(plane_index);
+    }
+}
+
 } // namespace pp::panopainter