[cmake] Declare renderer backend as public (#3717)

bin/CMakeLists.txt

@@ -56,7 +56,3 @@ install(TARGETS mbgl-offline mbgl-render RUNTIME DESTINATION bin)
 # ${PROJECT_SOURCE_DIR} )
 #
 # add_test(NAME mbgl-render-tool-test COMMAND mbgl-render WORKING_DIRECTORY ${PROJECT_SOURCE_DIR} )
-
-if(MLN_WITH_OPENGL)
-    target_compile_definitions(mbgl-render PRIVATE "MLN_RENDER_BACKEND_OPENGL=1")
-endif()