Add can_change for console variables

See merge request STJr/SRB2!2196

ZDoom translations

See merge request STJr/SRB2!2190

Fix IPv6 bans incorrectly banning IPv4 address

See merge request STJr/SRB2!2265

Added support for using textures as conversation icons
Implemented V_DrawTexture
Misc. cleanup

Split code out of m_misc.c into m_writebuffer.c
Turned text buffer variables into a struct
Use F_DrawDialogText for speaker name and choices strings
Support multiline strings and color commands in page 'name' and choices 'text'

Make Tokenizer_Read capture quotation marks as individual tokens
Make DIALOGUE parser more strict about capturing strings
Make DIALOGUE parser treat identifiers and keywords as case-insensitive
Support multi-line page dialogs

Now text speed 0 is used to type text immediately.
Text speed >= 1 works as it did before, but offset by one.