Add two more DirectWrite rendering modes

[Teddy]

On previous versions of Firefox from early to late 2010s, they were able to render fonts to unhinted greyscale outlines (DWRITE_RENDERING_MODE1_OUTLINE) when gfx.font_rendering.cleartype_params.rendering_mode is set to 6. Unfortunately, it no longer works since 2019. Please bring back the ability to use unhinted greyscale outlines when the parameter is set to 6, and also add the seventh parameter for gfx.font_rendering.cleartype_params.rendering_mode, in order to allow the ability to render DWRITE_RENDERING_MODE1_NATURAL_SYMMETRIC_DOWNSAMPLED on fonts. I hope they should be implemented in future releases of Firefox.

[Jon]

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