gencpp: clean code; move more things to preset

cmd/codegen/cimgui-go-preset.json

@@ -76,8 +76,15 @@
                 "TextEditor_GetText": "TextEditor_GetText_alloc"
         },
         "DefaultArgReplace": {
-                "FLT_MIN": "igGET_FLT_MIN()",
-                "FLT_MAX": "igGET_FLT_MAX()"
+                "FLT_MIN":    "igGET_FLT_MIN()",
+                "FLT_MAX":    "igGET_FLT_MAX()",
+                "nullptr":    "0",
+                "NULL":       "0",
+                "((void*)0)": "0"
+        },
+        "DefaultArgArbitraryValue": {
+                "text_end":   0,
+                "text_end_": "0"
         },
         "ExtraCGOPreamble": [
                 "#include \"../imgui/extra_types.h\""

cmd/codegen/gencpp.go

@@ -239,16 +239,8 @@ extern "C" {
 					v = string(r)
 				}
 
-				if v == "((void*)0)" {
-					v = "NULL"
-				}
-
-				if v == "nullptr" || v == "NULL" {
-					v = "0"
-				}
-
-				if k == "text_end" || k == "text_end_" {
-					v = "0"
+				if r, ok := ctx.preset.DefaultArgArbitraryValue[CIdentifier(k)]; ok {
+					v = string(r)
 				}
 
 				if strings.Contains(invocationStmt, ","+k) {

cmd/codegen/presets.go

@@ -39,6 +39,10 @@ type Preset struct {
 	// cimgui-go uses this to change FLT_MIN with igGet_FLT_MIN()
 	// NOTE: Iterated randomly!
 	DefaultArgReplace map[CIdentifier]CIdentifier
+	// DefaultArgArbitraryValue is similar to the above DefaultArgReplace, but
+	// associates default arg name with any arbitrary value.
+	// cimgui-go uses this to set text_end to 0
+	DefaultArgArbitraryValue map[CIdentifier]CIdentifier
 	// ExtraCGOPreamble allows to specify additional C code elements in Go files.
 	// For example could be used to include extra files.
 	// For ease of use, its in form of []string. These lines will be merged and prefixed (if appliable) with '//'