Fix vgl quad's renderer context.

Use the proper context for the vgl quads rather than the generic
context.  This makes cleaning up the layer's context more robust.  This
also tries to lose a context as soon as possible, if the browser
supports doing so.

src/gl/quadFeature.js

@@ -147,7 +147,7 @@ var gl_quadFeature = function (arg) {
    * Build this feature.
    */
   this._build = function () {
-    var mapper, mat, prog, srctex, unicrop, geom;
+    var mapper, mat, prog, srctex, unicrop, geom, context;
 
     if (!m_this.position()) {
       return;
@@ -174,10 +174,11 @@ var gl_quadFeature = function (arg) {
       unicrop = new vgl.uniform(vgl.GL.FLOAT_VEC2, 'crop');
       unicrop.set([1.0, 1.0]);
       prog.addUniform(unicrop);
+      context = m_this.renderer()._glContext();
       prog.addShader(vgl.getCachedShader(
-          vgl.GL.VERTEX_SHADER, vgl.GL, vertexShaderImageSource));
+          vgl.GL.VERTEX_SHADER, context, vertexShaderImageSource));
       prog.addShader(vgl.getCachedShader(
-          vgl.GL.FRAGMENT_SHADER, vgl.GL, fragmentShaderImageSource));
+          vgl.GL.FRAGMENT_SHADER, context, fragmentShaderImageSource));
       mat.addAttribute(prog);
       mat.addAttribute(new vgl.blend());
       /* This is similar to vgl.planeSource */
@@ -214,9 +215,10 @@ var gl_quadFeature = function (arg) {
       prog.addUniform(new vgl.projectionUniform('projectionMatrix'));
       prog.addUniform(new vgl.floatUniform('opacity', 1.0));
       prog.addUniform(new vgl.uniform(vgl.GL.FLOAT_VEC3, 'vertexColor'));
+      context = m_this.renderer()._glContext();
       prog.addShader(vgl.getCachedShader(
-          vgl.GL.VERTEX_SHADER, vgl.GL, vertexShaderColorSource));
-      prog.addShader(vgl.utils.createFragmentShader(vgl.GL));
+          vgl.GL.VERTEX_SHADER, context, vertexShaderColorSource));
+      prog.addShader(vgl.utils.createFragmentShader(context));
       mat.addAttribute(prog);
       mat.addAttribute(new vgl.blend());
       /* This is similar to vgl.planeSource */

src/gl/vglRenderer.js

@@ -178,6 +178,19 @@ var vglRenderer = function (arg) {
     }
   };
 
+  /**
+   * Get the GL context for this renderer.
+   *
+   * @returns {WebGLRenderingContext} The current context.  If unavailable,
+   *    falls back to the vgl generic context.
+   */
+  this._glContext = function () {
+    if (m_viewer && m_viewer.renderWindow()) {
+      return m_viewer.renderWindow().context();
+    }
+    return vgl.GL;
+  };
+
   /**
    * Exit.
    */
@@ -187,9 +200,14 @@ var vglRenderer = function (arg) {
     if (m_viewer) {
       var renderState = new vgl.renderState();
       renderState.m_renderer = m_viewer;
-      renderState.m_context = m_viewer.renderWindow().context();
+      renderState.m_context = this._glContext();
       m_viewer.exit(renderState);
+      if (this._glContext() !== vgl.GL && this._glContext().getExtension('WEBGL_lose_context') && this._glContext().getExtension('WEBGL_lose_context').loseContext) {
+        this._glContext().getExtension('WEBGL_lose_context').loseContext();
+      }
     }
+    // make sure we clear shaders associated with the generate context, too
+    vgl.clearCachedShaders(vgl.GL);
     m_viewer = null;
     s_exit();
   };