Add MGL entry points and port spinning_cube to use them

mojo/public/c/gpu/MGL/mgl.h

Index: mojo/public/c/gpu/MGL/mgl.h
diff --git a/mojo/public/c/gpu/MGL/mgl.h b/mojo/public/c/gpu/MGL/mgl.h
new file mode 100644
index 0000000000000000000000000000000000000000..5c91b0a2a6eae656015adaeb97a115dd5eb164e6
--- /dev/null
+++ b/mojo/public/c/gpu/MGL/mgl.h
@@ -0,0 +1,63 @@
+// Copyright 2015 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.
+
+#ifndef MOJO_PUBLIC_C_GPU_MGL_MGL_H_
+#define MOJO_PUBLIC_C_GPU_MGL_MGL_H_
+
+// Note: This header should be compilable as C.

[viettrungluu, 2015/08/12 02:48:10]

nit: I think I prefer this comment to be above the include guards.

+
+#include <stdint.h>
+
+#include "mojo/public/c/environment/async_waiter.h"

[viettrungluu, 2015/08/12 02:48:10]

You don't actually need to include this here. <shrug>

+#include "mojo/public/c/gpu/MGL/mgl_types.h"
+#include "mojo/public/c/system/types.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef uint32_t MGLOpenGLAPIVersion;
+
+// OpenGL ES 2.0
+#define MGL_API_VERSION_GLES2 ((MGLOpenGLAPIVersion)1)
+// OpenGL ES 3.0
+#define MGL_API_VERSION_GLES3 ((MGLOpenGLAPIVersion)2)
+// OpenGL ES 3.1
+#define MGL_API_VERSION_GLES31 ((MGLOpenGLAPIVersion)3)
+
+#define MGL_NO_CONTEXT ((MGLContext)0)
+
+// Creates a context at the given API version or returns MGL_NO_CONTEXT.
+// |command_buffer_handle| must be a command buffer message pipe handle from
+// the Gpu service or another source. The callee takes ownership of this
+// handle.
+// |share_group| specifies the share group to create this context in.
+// If this is MGL_NO_CONTEXT a new share group will be created for this context.
+// |lost_callback|, if not null, will be invoked when the context is lost.
+// |async_waiter| must be a pointer to a MojoAsyncWaiter implementation that is
+// usable from any thread the returned MGLContext will be used from
+// for as long as the context exists.
+MGLContext MGLCreateContext(MGLOpenGLAPIVersion version,
+                            MojoHandle command_buffer_handle,
+                            MGLContext share_group,
+                            MGLContextLostCallback lost_callback,
+                            void* lost_callback_closure,
+                            const struct MojoAsyncWaiter* async_waiter);
+void MGLDestroyContext(MGLContext context);
+
+// Makes |context| the current MGLContext for the calling thread. Calling with
+// MGL_NO_CONTEXT clears the current context.  Returns 0 if successful, nonzero

[viettrungluu, 2015/08/12 02:48:10]

Do you really want to return 0 for success? Is the nonzero value some sort of
error code?

(Or do you want this to be a poor man's bool? eglMakeCurrent() returns "true" on
success and "false" on failure.)

+// if an error was encountered.
+// If this does not return success the context is lost.

[viettrungluu, 2015/08/12 02:48:10]

Which context is lost? |context|?

Also, what happens to the current context (i.e., the one that was current before
this call) on failure? (Possible answers: a) This is undefined. b) The context
what was current will always no longer be current. c) It'll still be current,
unless it's the same as |context|.)

+int MGLMakeCurrent(MGLContext context);
+
+// Returns the currently bound context for the calling thread or MGL_NO_CONTEXT
+// if there is none.
+MGLContext MGLGetCurrentContext(void);
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
+#endif  // MOJO_PUBLIC_C_GPU_MGL_MGL_H_