kwin: Add a new GLVertexBuffer::draw() method

Expose bindArrays(), unbindArrays() and add a draw() method that takes
an offset and a count. This makes it possible to upload geometry, call
bindArrays(), and then call draw() multiple times to draw different
subsets of the uploaded geometry.

libkwineffects/kwinglutils.cpp

@@ -1600,18 +1600,31 @@ void GLVertexBuffer::render(GLenum primitiveMode)
 void GLVertexBuffer::render(const QRegion& region, GLenum primitiveMode, bool hardwareClipping)
 {
     d->bindArrays();
+    draw(region, primitiveMode, 0, d->vertexCount, hardwareClipping);
+    d->unbindArrays();
+}
 
+void GLVertexBuffer::bindArrays()
+{
+    d->bindArrays();
+}
+
+void GLVertexBuffer::unbindArrays()
+{
+    d->unbindArrays();
+}
+
+void GLVertexBuffer::draw(const QRegion &region, GLenum primitiveMode, int first, int count, bool hardwareClipping)
+{
     if (!hardwareClipping) {
-        glDrawArrays(primitiveMode, 0, d->vertexCount);
+        glDrawArrays(primitiveMode, first, count);
     } else {
         // Clip using scissoring
         foreach (const QRect &r, region.rects()) {
             glScissor(r.x(), displayHeight() - r.y() - r.height(), r.width(), r.height());
-            glDrawArrays(primitiveMode, 0, d->vertexCount);
+            glDrawArrays(primitiveMode, first, count);
         }
     }
-
-    d->unbindArrays();
 }
 
 bool GLVertexBuffer::isSupported()

libkwineffects/kwinglutils.h

@@ -707,6 +707,21 @@ public:
      */
     void unmap();
 
+    /**
+     * Binds the vertex arrays to the context.
+     */
+    void bindArrays();
+
+    /**
+     * Disables the vertex arrays.
+     */
+    void unbindArrays();
+
+    /**
+     * Draws count vertices beginning with first.
+     */
+    void draw(const QRegion &region, GLenum primitiveMode, int first, int count, bool hardwareClipping = false);
+
     /**
      * Renders the vertex data in given @a primitiveMode.
      * Please refer to OpenGL documentation of glDrawArrays or glDrawElements for allowed