cogl-pixel-buffer: Add a fallback path

First, let's add a new public feature called, surprisingly,
COGL_FEATURE_PBOS to check the availability of PBOs and provide a
fallback path when running on older GL implementations or on OpenGL ES

In case the underlying OpenGL implementation does not provide PBOs, we
need a fallback path (a malloc'ed buffer). The CoglPixelBufer
constructors will instanciate a subclass of CoglBuffer that handles
map/unmap and set_data() with a malloc'ed buffer.

The public feature is useful to check before using set_data() on a
buffer as it will mean doing a memcpy() when not supporting PBOs (in
that case, it's better to create the texture directly instead of using a
CoglBuffer).

cogl/driver/gl/cogl-feature-functions.h

@@ -99,6 +99,11 @@ COGL_FEATURE_FUNCTION (void, glRenderbufferStorageMultisample,
                         GLsizei               width,
                         GLsizei               height))
 COGL_FEATURE_END ()
+COGL_FEATURE_BEGIN (read_pixels_async, 2, 1,
+                    "EXT\0",
+                    "pixel_buffer_object\0",
+                    COGL_FEATURE_PBOS)
+COGL_FEATURE_END ()
 
 /* The function names in OpenGL 2.0 are different so we can't easily
    just check for GL 2.0 */