Implement response to DECRQM for DECECM as "permanently reset" #12505

The physical terminals most of us are emulating, namely VT100 and VT220s, don't deal with this, since they do not have color support as mentioned in the above sources.

Note that there are a couple of hardware terminals with color support - most notably the VT525 - and it actually has a mode that allows apps to control this behaviour: DECECM (Erase Color Mode). As defined in the programmer documentation, "it controls the background color used when text is erased or new text is scrolled on to the screen". When set, these operations fill with the default attributes (compatible with monochrome terminals). When reset, they fill with the active rendition colors (which is compatible with most modern color terminal emulators). The factory default is reset, i.e. it fills with the active colors.

While DECECM isn't actually listed as a conformance requirement for the Color Text Extension, I think it would be a good idea for color terminals to a least report how they are implementing that mode in response to a DECRQM query (i.e. permanently reset if you always fill with the active colors, or permanently set if you always fill with the default attributes). That way apps can potentially adjust their behavior to match. Ideally I'd like to see more terminals fully support the mode, though, so apps can turn it on if they don't want the color background fill when scrolling.

Originally posted by @j4james in #12497 (reply in thread)