Improve anti aliased text rendering

[alexfaria]

Hi @bitbank2 - this library is amazing and I am loving your work

I am using your library to control a LILYGO T5 E-Paper S3 Pro and I'm displaying some anti aliased text

I've encountered some small visual bugs and I'm also hoping to add dark mode support

Hopefully this PR helps to explain what the problem is and possible fixes

• Inverted the grayscale color mapping when the background is black (bbepWriteStringCustom, src/bb_ep_gfx.inl).
• Added a check to only draw pixels that are not the background color, which prevents drawing transparent pixels and avoids overlapping pixels from previous characters (important for italic fonts) (bbepWriteStringCustom, src/bb_ep_gfx.inl).
• Cleared the line buffer before decoding each line to prevent artifacts from previous data (bbepWriteStringCustom, src/bb_ep_gfx.inl).
• Applied a mask to the last byte of the line buffer when the width is not a multiple of 8, improving edge rendering accuracy (bbepWriteStringCustom, src/bb_ep_gfx.inl).

Before - note the vertical edges and the clipped italic text

Before - white text on black background

After - fixed vertical edges and clipped text

After - inverted grays if background is black