Shrink content and improve rendering performance

Summarize the definition of Content.
Unification of content variable names.

oviewer/content.go

@@ -36,6 +36,9 @@ var SpaceContent = content{
 	style: tcell.StyleDefault,
 }
 
+// Shrink is a character that represents a shrunk column.
+var Shrink rune = '…'
+
 // ShrinkContent is a content that represents a shrunk column.
 var ShrinkContent = content{
 	mainc: Shrink,

oviewer/draw.go

@@ -194,11 +194,11 @@ func (root *Root) drawNoWrapLine(y int, startX int, lN int, lineC LineC) (int, i
 			root.clearEOL(root.scr.startX+x, y, lineC.eolStyle)
 			break
 		}
-		content := DefaultContent
+		c := DefaultContent
 		if startX+x >= 0 {
-			content = lineC.lc[startX+x]
+			c = lineC.lc[startX+x]
 		}
-		root.Screen.SetContent(root.scr.startX+x, y, content.mainc, content.combc, content.style)
+		root.Screen.SetContent(root.scr.startX+x, y, c.mainc, c.combc, c.style)
 	}
 	lN++
 

oviewer/oviewer.go

@@ -331,8 +331,6 @@ const (
 	nameGeneral string = "general"
 )
 
-var Shrink rune = '…'
-
 var (
 	// ErrOutOfRange indicates that value is out of range.
 	ErrOutOfRange = errors.New("out of range")