【igl nanovg part-1】shell | mac | add stencil buffer (#216)

Summary:
This is a prerequisite pull request for igl nanovg(#213).

Pull Request resolved: #216

Reviewed By: EricGriffith

Differential Revision: D66678263

Pulled By: corporateshark

fbshipit-source-id: 9c7713d391fde908d28206d5af77011f154ffabc

shell/mac/ViewController.mm

@@ -196,6 +196,11 @@ - (void)loadView {
 
 #if IGL_BACKEND_OPENGL
   case igl::BackendFlavor::OpenGL: {
+    const bool enableStencilBuffer =
+        config_.depthTextureFormat == igl::TextureFormat::S8_UInt_Z24_UNorm ||
+        config_.depthTextureFormat == igl::TextureFormat::S_UInt8;
+    const NSOpenGLPixelFormatAttribute stencilSize = enableStencilBuffer ? 8 : 0;
+
     NSOpenGLPixelFormat* pixelFormat;
     if (config_.backendVersion.majorVersion == 4 && config_.backendVersion.minorVersion == 1) {
       static NSOpenGLPixelFormatAttribute attributes[] = {
@@ -211,6 +216,8 @@ - (void)loadView {
           32,
           NSOpenGLPFADepthSize,
           24,
+          NSOpenGLPFAStencilSize,
+          stencilSize,
           NSOpenGLPFAOpenGLProfile,
           NSOpenGLProfileVersion4_1Core,
           0,
@@ -232,6 +239,8 @@ - (void)loadView {
           32,
           NSOpenGLPFADepthSize,
           24,
+          NSOpenGLPFAStencilSize,
+          stencilSize,
           NSOpenGLPFAOpenGLProfile,
           NSOpenGLProfileVersion3_2Core,
           0,
@@ -252,6 +261,8 @@ - (void)loadView {
           32,
           NSOpenGLPFADepthSize,
           24,
+          NSOpenGLPFAStencilSize,
+          stencilSize,
           NSOpenGLPFAOpenGLProfile,
           NSOpenGLProfileVersionLegacy,
           0,