Use updateData instead of re-creating buffers for repopulated paint a…

…rrays.

Fixes issue #6839 (memory leak on frequent calls to setFeatureState). The leak was caused by mismatched calls to "createVertexBuffer" and "destroy".
Improves performance of PaintStates benchmark. The performance improvement is presumably because we're able to take advantage of the DYNAMIC_DRAW option intended for frequently updated buffers.

src/data/program_configuration.js

@@ -185,7 +185,11 @@ class SourceExpressionBinder<T> implements Binder<T> {
 
     upload(context: Context) {
         if (this.paintVertexArray && this.paintVertexArray.arrayBuffer) {
-            this.paintVertexBuffer = context.createVertexBuffer(this.paintVertexArray, this.paintVertexAttributes, this.expression.isStateDependent);
+            if (this.paintVertexBuffer && this.paintVertexBuffer.buffer) {
+                this.paintVertexBuffer.updateData(this.paintVertexArray);
+            } else {
+                this.paintVertexBuffer = context.createVertexBuffer(this.paintVertexArray, this.paintVertexAttributes, this.expression.isStateDependent);
+            }
         }
     }
 
@@ -280,7 +284,11 @@ class CompositeExpressionBinder<T> implements Binder<T> {
 
     upload(context: Context) {
         if (this.paintVertexArray && this.paintVertexArray.arrayBuffer) {
-            this.paintVertexBuffer = context.createVertexBuffer(this.paintVertexArray, this.paintVertexAttributes, this.expression.isStateDependent);
+            if (this.paintVertexBuffer && this.paintVertexBuffer.buffer) {
+                this.paintVertexBuffer.updateData(this.paintVertexArray);
+            } else {
+                this.paintVertexBuffer = context.createVertexBuffer(this.paintVertexArray, this.paintVertexAttributes, this.expression.isStateDependent);
+            }
         }
     }