Revert "vo_opengl: disable alpha by default"

This reverts commit a33b46194c.

It turns out FFmpeg really considers this a bug, and fixed it by making
the decoder output the correct pixel format.

Fixes #1565. Reverts the fix #1528, though it should work fine with
a recent git master FFmpeg.

DOCS/man/vo.rst

@@ -562,7 +562,7 @@ Available video output drivers are:
         Sizes must be a power of two, and 512 at most.
 
     ``alpha=<blend|yes|no>``
-        Decides what to do if the input has an alpha component (default: no).
+        Decides what to do if the input has an alpha component (default: blend).
 
         blend
             Blend the frame against a black background.

video/out/gl_video.c

@@ -325,7 +325,7 @@ const struct gl_video_opts gl_video_opts_def = {
     .scalers = { "bilinear", "bilinear" },
     .scaler_params = {{NAN, NAN}, {NAN, NAN}},
     .scaler_radius = {3, 3},
-    .alpha_mode = 0,
+    .alpha_mode = 2,
     .background = {0, 0, 0, 255},
     .gamma = 1.0f,
 };
@@ -343,7 +343,7 @@ const struct gl_video_opts gl_video_opts_hq_def = {
     .dscaler = "mitchell",
     .scaler_params = {{NAN, NAN}, {NAN, NAN}},
     .scaler_radius = {3, 3},
-    .alpha_mode = 0,
+    .alpha_mode = 2,
     .background = {0, 0, 0, 255},
     .gamma = 1.0f,
 };