xrRender: fixed OpenGL errors queue behavior (close #647)

Created by: eanmos

I've managed to reproduce the bug. When the green screen appears the following message is seen in log:

	intro_start intro_game
	Invalid video stream: 0x500

This leads to file xrRenderGL/glSH_Texture.cpp:

            GLenum err = glGetError();
            if (err != GL_NO_ERROR)
            {
                Msg("Invalid video stream: 0x%x", err);
                xr_delete(pTheora);
                pSurface = 0;
            }

However the actual errors appears a few lines above:

	Preload();

After this function call the OpenGL errors queue contains an 0x500 error. If we check what is happening in the end of this function:

	void CTexture::Preload()
	{
	    m_bumpmap = RImplementation.Resources->m_textures_description.GetBumpName(cName);
	    m_material = RImplementation.Resources->m_textures_description.GetMaterial(cName);

	    if (glGetError() != GL_NO_ERROR)
		    Msg("Warning: can't preload texture %s", cName.c_str());
	}

we will get

	Warning: can't preload texture water\water_flowing_nmap

So, I guess this texture is just broken.

This check actually fixes the bug because it clears the OpenGL errors queue and the following code could work correctly.