Emscripten staging buffer support broken

[crlf0710]

Currently the staging texture support is implemented based on glMapBuffer, which works fine on desktop OpenGL, but not WebGL. It will trigger error at the emscripten level that says: "glMapBufferRange is only supported when access is MAP_WRITE|INVALIDATE_BUFFER", while the actual parameter passed is actually GL_MAP_READ_BIT. Maybe some glGetBufferSubData based implementation will work.