Engine: refactor passing text_color into display_main()

[ivan-mogilko]

• Replaced DISPLAYTEXT_x macros with DisplayTextType enum;
• Got rid of confusing asspch parameter which combines text color with the "text style" by applying a sign. Instead have separate text_color and a style parameter of DisplayTextStyle enum.
• Also added DisplayTextShrink enum for the allow_shrink parameter.
• Grouped them into DisplayTextLooks struct.

[ericoporto]

Great work! The display stuff is one of the parts of the engine where there is still a lot of cruft from the original code so this refactor looks great.

I think in similar fashion, only aschar rename to char_id and allowShrink being made an enum (with a rename?) would be things in similar changes I would make to this code. Obviously not necessary here just they are things I noticed when looking the code adjacent to the git diff one in the web interface.

[ivan-mogilko]

Changed to another variant, with enums and a struct to group them instead of flags.

Found and fixed one case when overlay was passing a negative color value to force certain style....

[ericoporto]

I tested this with a few of my own test games and also the shivah, and afaict all is still working alright.