Wayland: Fix window opacity on older systems

On systems lacking the EGL_EXT_present_opaque extension, some
compositors treat any buffer with an alpha channel as per-pixel
transparent.

This commit ignores any EGLConfig with an alpha channel if the extension
is missing and the window is created with GLFW_TRANSPARENT_FRAMEBUFFER
set to false.

This is technically not a breaking change since GLFW_ALPHA_BITS is not
a hard constraint, but it is still going to inconvenience anyone using
the framebuffer alpa channel to store other kinds of data.

Related to #1895

(cherry picked from commit ef6c9d8b4f)

src/egl_context.c

@@ -172,6 +172,18 @@ static GLFWbool chooseEGLConfig(const _GLFWctxconfig* ctxconfig,
         u->depthBits = getEGLConfigAttrib(n, EGL_DEPTH_SIZE);
         u->stencilBits = getEGLConfigAttrib(n, EGL_STENCIL_SIZE);
 
+#if defined(_GLFW_WAYLAND)
+        // NOTE: The wl_surface opaque region is no guarantee that its buffer
+        //       is presented as opaque, if it also has an alpha channel
+        // HACK: If EGL_EXT_present_opaque is unavailable, ignore any config
+        //       with an alpha channel to ensure the buffer is opaque
+        if (!_glfw.egl.EXT_present_opaque)
+        {
+            if (!desired->transparent && u->alphaBits > 0)
+                continue;
+        }
+#endif // _GLFW_WAYLAND
+
         u->samples = getEGLConfigAttrib(n, EGL_SAMPLES);
         u->doublebuffer = desired->doublebuffer;