cmake: testshader needs -sLEGACY_GL_EMULATION on Emscripten

test/CMakeLists.txt

@@ -235,6 +235,9 @@ if(OPENGL_FOUND)
         target_link_libraries(testgl2 ${OPENGL_gl_LIBRARY})
     endif()
 endif()
+if(EMSCRIPTEN)
+    target_link_libraries(testshader -sLEGACY_GL_EMULATION)
+endif()
 
 file(GLOB RESOURCE_FILES *.bmp *.wav *.hex moose.dat utf8.txt)
 file(COPY ${RESOURCE_FILES} DESTINATION ${CMAKE_CURRENT_BINARY_DIR})