vo_opengl: add alpha output

Allows playing video with alpha information on X11, as long as the video
contains alpha and the window manager does compositing. See vo.rst.

Whether a window can be transparent is decided by the choice of the X
Visual used for window creation. Unfortunately, there's no direct way to
request such a Visual through the GLX or the X API, and use of the
XRender extension is required to find out whether a Visual implies a
framebuffer with alpha used by XRender (see for example [1]). Instead of
depending on the XRender wrapper library (which would require annoying
configure checks, even though XRender is virtually always supported),
use a simple heuristics to find out whether a Visual has alpha. Since
getting it wrong just means an optional feature will not work as
expected, we consider this ok.

[1] http://stackoverflow.com/questions/4052940/how-to-make-an-opengl-
rendering-context-with-transparent-background/9215724#9215724

1 files changed, 1 insertions, 0 deletions

diff --git a/video/out/gl_video.h b/video/out/gl_video.h
index d4de02ba0f..985db9b9cc 100644
--- a/video/out/gl_video.h
+++ b/video/out/gl_video.h
@@ -40,6 +40,7 @@ struct gl_video_opts {
     int dither_depth;
     int fbo_format;
     int stereo_mode;
+    int enable_alpha;
 };
 
 extern const struct m_sub_options gl_video_conf;