formats: Fix readback condition for depth buffers

The old implementation of vrend_add_formats used to check color readback format even if a depth buffer is bound. As no texture is attached to the color buffer for such a FBO, it makes glGetIntegerv with GL_IMPLEMENTATION_COLOR_READ_TYPE fail, and the error can trigger the GL_NO_ERROR assertion in the function.

Check only if the depth buffer can be read in such a case.

Signed-off-by: Akihiko Odaki akihiko.odaki@gmail.com