Speed up text layout by not rendering glyphs

[nulano]

For #4651

I would assume the advance value only depends on hinting, which should probably run without the FT_LOAD_RENDER flag. In fact, the FreeType docs mention the flag FT_LOAD_BITMAP_METRICS_ONLY which seems appropriate here and conflicts with FT_LOAD_RENDER. https://www.freetype.org/freetype2/docs/reference/ft2-base_interface.html#ft_load_xxx

While the FT_LOAD_BITMAP_METRICS_ONLY will likely be ignored due to FT_LOAD_NO_BITMAP, it should serve as a reminder for when #891 is resolved properly. (I know the solution, I intend to submit a PR later. Essentially, Pillow should check the bitdepth of the returned bitmap instead of guessing.) Edit: Removed for now as it is not supported on all FreeType versions.

Testing I get the following results (in s) on my Windows 10 laptop with the code below.

7.1.2 (PyPI)	master (local compile)	PR (local compile)
0.088	0.0895	0.079

    from timeit import repeat

    print(
        'getsize() (x100): ',
        min(repeat(
            'font.getsize("Hello world")',
            'from PIL import ImageFont; font = ImageFont.truetype("Tests/fonts/FreeMono.ttf", 20, layout_engine=ImageFont.LAYOUT_BASIC)',
            number=100, repeat=30
        )),
    )

[radarhere]

Thanks!

[hugovk]

Test on macOS, also improved:

7.1.2 (PyPI)	master before merge (local compile)	master after merge (local compile)
0.035	0.015	0.011