Route VR checkerboard setup through helper

src/app_vr.cpp

@@ -3,6 +3,7 @@
 #include <cstdint>
 
 #include "app.h"
+#include "legacy_canvas_draw_merge_services.h"
 #include "legacy_canvas_stroke_composite_services.h"
 #include "legacy_canvas_stroke_erase_services.h"
 #include "legacy_ui_gl_dispatch.h"
@@ -274,9 +275,11 @@ void App::vr_draw(const glm::mat4& proj, const glm::mat4& camera, const glm::mat
             canvas->m_canvas->m_plane_transform[plane_index] *
             glm::translate(glm::vec3(0, 0, -1));
 
-        ShaderManager::use(kShader::Checkerboard);
-        ShaderManager::u_int(kShaderUniform::Colorize, false);
-        ShaderManager::u_mat4(kShaderUniform::MVP, plane_mvp);
+        pp::panopainter::setup_legacy_canvas_draw_merge_checkerboard_shader(
+            pp::panopainter::LegacyCanvasDrawMergeCheckerboardUniforms {
+                .mvp = plane_mvp,
+                .colorize = false,
+            });
         m_face_plane.draw_fill();
     }