ui/tui: highlighting refactor

Make HlAttr contain highlighting state for both color modes (cterm and rgb).
This allows us to implement termguicolors completely in the TUI.

Simplify some logic duplicated between ui.c and screen.c. Also avoid
some superfluous highlighting reset events.

src/nvim/syntax_defs.h

@@ -68,21 +68,4 @@ struct syn_state {
                                  * may have made the state invalid */
 };
 
-// Structure shared between syntax.c, screen.c
-typedef struct attr_entry {
-  int16_t rgb_ae_attr, cterm_ae_attr;  // HL_BOLD, etc.
-  RgbValue rgb_fg_color, rgb_bg_color, rgb_sp_color;
-  int cterm_fg_color, cterm_bg_color;
-} attrentry_T;
-
-#define ATTRENTRY_INIT { \
-  .rgb_ae_attr = 0, \
-  .cterm_ae_attr = 0, \
-  .rgb_fg_color = -1, \
-  .rgb_bg_color = -1, \
-  .rgb_sp_color = -1, \
-  .cterm_fg_color = 0, \
-  .cterm_bg_color = 0, \
-}
-
 #endif // NVIM_SYNTAX_DEFS_H