window: egl: speculatively relax min alpha size

In refs: https://github.com/wez/wezterm/issues/240 there are a number
of configurations that report 0 for the alpha size and where we are
unable to otherwise find a working config.

This is a speculative commit to releax the alpha channel size to
basically anything available and see if that helps.

window/src/egl.rs

@@ -365,7 +365,7 @@ impl GlState {
                     // Request at least 8bpc, 24bpp.  The implementation may
                     // return a context capable of more than this
                     ffi::ALPHA_SIZE,
-                    8,
+                    0,
                     ffi::RED_SIZE,
                     8,
                     ffi::GREEN_SIZE,