feat(preserveAspectRatio): added option to preserve aspect ratio

When set to true and both width and height are specified, they are interpreted as the minimum width and minimum height, respectively.
If set to true with only the width specified, the height will be automatically determined while preserving the aspect ratio, and vice versa.

fixes #88, #189

src/graphics.ts

@@ -13,6 +13,8 @@ export class Graphics {
 
   private height = 512;
 
+  private preserveAspectRatio = false;
+
   private density = 72;
 
   private savePath = "./";
@@ -37,6 +39,7 @@ export class Graphics {
   }
 
   public gmBaseCommand(stream: fs.ReadStream, filename: string): gm.State {
+    // console.log(JSON.stringify({ width: this.width, height: this.height, preserveAspectRatio: this.preserveAspectRatio, density: this.density }))
     return this.gm(stream, filename)
       .density(this.density, this.density)
       .resize(this.width, this.height, this.preserveAspectRatio ? '^' : '!')
@@ -136,7 +139,13 @@ export class Graphics {
 
   public setSize(width: number, height?: number): Graphics {
     this.width = width;
-    this.height = !!height ? height : width;
+    this.height = this.preserveAspectRatio || !!height ? height : width;
+
+    return this;
+  }
+
+  public setPreserveAspectRatio(preserveAspectRatio: boolean): Graphics {
+    this.preserveAspectRatio = preserveAspectRatio;
 
     return this;
   }
@@ -189,6 +198,7 @@ export class Graphics {
       format:       this.format,
       width:        this.width,
       height:       this.height,
+      preserveAspectRatio: this.preserveAspectRatio,
       density:      this.density,
       savePath:     this.savePath,
       saveFilename: this.saveFilename,