Implement HiDPI support in Pepper dev interface

ppapi/api/private/ppb_graphics_2d_private.idl

+/* Copyright (c) 2012 The Chromium Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+/* This file contains the <code>PPB_Graphics2D_Private</code> interface. */
+label Chrome {
+  M22 = 0.1
+};
+
+/* PPB_Graphics2D_Private interface */
+interface PPB_Graphics2D_Private {
+  /**
+   * Create() creates a 2D graphics context. The returned graphics context will
+   * not be bound to the module instance on creation (call BindGraphics() on
+   * the module instance to bind the returned graphics context to the module
+   * instance).
+   *
+   * @param[in] instance The module instance.
+   * @param[in] size The size of the graphic context.
+   * @param[in] is_always_opaque Set the <code>is_always_opaque</code> flag to
+   * <code>PP_TRUE</code> if you know that you will be painting only opaque
+   * data to this context. This option will disable blending when compositing
+   * the module with the web page, which might give higher performance on some
+   * computers.
+   *
+   * If you set <code>is_always_opaque</code>, your alpha channel should always
+   * be set to 0xFF or there may be painting artifacts. The alpha values
+   * overwrite the destination alpha values without blending when
+   * <code>is_always_opaque</code> is true.
+   *
+   * @param[in] scale The scaling factor between pixels in this context and
+   * DIPs. For rendering at device resolution, this will typically be equal to
+   * the device scale of the view resource passed to DidChangeView.
+   * For example, if the view resource passed to DidChangeView has a rectangle
+   * of (w=200, h=100) and a device scale of 2.0, one would call Create with a
+   * size of (w=400, h=200) and a scaler of 2.0 then treat each pixel in the
+   * context as a single device pixel.
+   *
+   * @return A <code>PP_Resource</code> containing the 2D graphics context if
+   * successful or 0 if unsuccessful.
+   */
+  PP_Resource Create(

[brettw, 2012/06/18 21:17:24]

This is a bit confusing because normally if I see an argument on a graphics
display that takes a scale factor and I pass 2, I would expect it to be drawn
twice as large. Your scale value is actually the inverse scale.

Can we instead use the "real" scale and then maybe on the getter for the scale
on the view we can say if you want to paint at device pixels, pass
1.0/GetDeviceScale() as the scale here.

I think it would be simpler to have a scale setter rather than a different
constructor:
  PP_Bool SetScale(PP_Resource resource, float_t scale).
Maybe we should also have a
  float_t GetScale(PP_Resource resource);

We should probably document that the scale can't be 0 or it will fail (and make
sure that this actually happens).

[Josh Horwich, 2012/06/19 02:01:04]

Done.

+      [in] PP_Instance instance,
+      [in] PP_Size size,
+      [in] PP_Bool is_always_opaque,
+      [in] float_t scale);
+};
+