[Bug 39852] 3DMark03: Mother Nature test: waterfall has no transparency if AllwaysOffScreen=disabled

https://bugs.winehq.org/show_bug.cgi?id=39852

--- Comment #3 from Sergey Isakov  ---
I understand.
There is no bug if set offscreen_rendering_mode = ORM_BACKBUFFER.
The bug is only if FBO+OffscreenDisabled.
In this case color_format = WINED3DFMT_B8G8R8X8_UNORM (no transparency).
In other cases {OffscreenRenderingMode, AlwaysOffscreen} the final color_format
will be WINED3DFMT_B8G8R8A8_UNORM (with transparency).

For backbuffer the correction is here:
~~~~
    /* In case of ORM_BACKBUFFER, make sure to request an alpha component for
     * X4R4G4B4/X8R8G8B8 as we might need it for the backbuffer. */
    if (wined3d_settings.offscreen_rendering_mode == ORM_BACKBUFFER)
    {
        auxBuffers = TRUE;

if (color_format->id == WINED3DFMT_B4G4R4X4_UNORM)
            color_format = wined3d_get_format(gl_info,
WINED3DFMT_B4G4R4A4_UNORM);
        else if (color_format->id == WINED3DFMT_B8G8R8X8_UNORM)
            color_format = wined3d_get_format(gl_info,
WINED3DFMT_B8G8R8A8_UNORM);
    }
~~~

As I see fbo+AllwaysOffscreen=disable also requires same correction, i.e.
unconditionally request
~~~
if (color_format->id == WINED3DFMT_B8G8R8X8_UNORM)
    color_format = wined3d_get_format(gl_info, WINED3DFMT_B8G8R8A8_UNORM);
~~~
What is the reason to this restriction? Is it cause other bugs?