Change ui_window_style_set and change the way cursorline is drawn

[infastin]

Changes:

• ui_window_style_set now accepts keep flag, that allows to not override non-default style values (fg and bg);
• ui_window_style_set_pos and win_style also accept the same flag;
• win:style and win:style_pos both accept the same keep flag, but it is optional;
• Now when drawing cursorline the CURSOR_LINE style won't override non-default values of other styles on the line.

This was mainly done so that CURSOR_LINE wouldn't override other styles, but plugins can potentially benefit from this as well.

[rnpnr]

Hi! I'm not sure I understand the point? CURSOR_LINE already doesn't override non-default styles if the user didn't request it. For example the zenburn theme only specifies the background for CURSOR_LINE so whatever foreground was set by the other rules remains unchanged when CURSOR_LINE is applied.

[infastin]

Hi! I'm not sure I understand the point? CURSOR_LINE already doesn't override non-default styles if the user didn't request it. For example the zenburn theme only specifies the background for CURSOR_LINE so whatever foreground was set by the other rules remains unchanged when CURSOR_LINE is applied.

If other styles on the same line have non-default background, then CURSOR_LINE will override them. That's is kind of the problem when using vis-sneak plugin, which sets CURSOR style on matches, which is then "overriden" by CURSOR_LINE when you are on the same line with the match.

Before:

After: