egl_helpers: request at least 8 alpha bits if necessary

Previously on wayland, it would result in an egl config with only 2 alpha
bits, which technically matches what was requested, but is not very useful.
Fixes #9862

video/out/opengl/egl_helpers.c

@@ -132,7 +132,7 @@ static bool create_context(struct ra_ctx *ctx, EGLDisplay display,
         EGL_RED_SIZE, 8,
         EGL_GREEN_SIZE, 8,
         EGL_BLUE_SIZE, 8,
-        EGL_ALPHA_SIZE, ctx->opts.want_alpha ? 1 : 0,
+        EGL_ALPHA_SIZE, ctx->opts.want_alpha ? 8 : 0,
         EGL_RENDERABLE_TYPE, rend,
         EGL_NONE
     };