EmuGL: Deliver every frame to a callback

To enable multi-touch on a tethered device, allow a callback to be
registered with the OpenGL renderer. On every frame, the framebuffer
is read into system memory and provided to the callback, so it can be
mirrored to the device.

This change is co-dependent on Idae3b026d52ed8dd666cbcdc3f3af80175c90ad3
in external/qemu.

Change-Id: I03c49bc55ed9e66ffb59462333181f77e7e46035

tools/emulator/opengl/host/include/libOpenglRender/render_api.h

@@ -22,6 +22,38 @@ extern "C" {
 
 #include "render_api_platform_types.h"
 
+/* If a function with this signature is passed to initOpenGLRenderer(),
+ * it will be called by the renderer just before each new frame is displayed,
+ * providing a copy of the framebuffer contents.
+ *
+ * The callback will be called from one of the renderer's threads, so will
+ * probably need synchronization on any data structures it modifies. The
+ * pixels buffer may be overwritten as soon as the callback returns; if it needs
+ * the pixels afterwards it must copy them.
+ *
+ * The pixels buffer is intentionally not const: the callback may modify the
+ * data without copying to another buffer if it wants, e.g. in-place RGBA to RGB
+ * conversion, or in-place y-inversion.
+ *
+ * Parameters are:
+ *   context        The pointer optionally provided when the callback was
+ *                  registered. The client can use this to pass whatever
+ *                  information it wants to the callback.
+ *   width, height  Dimensions of the image, in pixels. Rows are tightly packed;
+ *                  there is no inter-row padding.
+ *   ydir           Indicates row order: 1 means top-to-bottom order, -1 means
+ *                  bottom-to-top order.
+ *   format, type   Format and type GL enums, as used in glTexImage2D() or
+ *                  glReadPixels(), describing the pixel format.
+ *   pixels         The framebuffer image.
+ *
+ * In the first implementation, ydir is always -1 (bottom to top), format and
+ * type are always GL_RGBA and GL_UNSIGNED_BYTE, and the width and height will
+ * always be the same as the ones passed to initOpenGLRenderer().
+ */
+typedef void (*OnPostFn)(void* context, int width, int height, int ydir,
+                         int format, int type, unsigned char* pixels);
+
 // initLibrary - initialize the library and tries to load the corresponding
 //     GLES translator libraries. This function must be called before anything
 //     else to ensure that everything works. If it returns an error, then
@@ -53,8 +85,8 @@ int setStreamMode(int mode);
 // This function is *NOT* thread safe and should be called first
 // to initialize the renderer after initLibrary().
 //
-bool initOpenGLRenderer(int width, int height, int portNum);
-
+bool initOpenGLRenderer(int width, int height, int portNum,
+                        OnPostFn onPost, void* onPostContext);
 
 //
 // createOpenGLSubwindow -