gtk/opengl: print an error when OpenGL version is too old

#1123 added a warning when the OpenGL version is too old, but
it is never used because GTK enforces the version set with
gl_area.setRequiredVersion() before prepareContext() is called:
we end up with a generic "failed to make GL context" error:
warning(gtk_ghostty_surface): failed to make GL context current: Unable to create a GL context
warning(gtk_ghostty_surface): this error is almost always due to a library, driver, or GTK issue
warning(gtk_ghostty_surface): this is a common cause of this issue: https://ghostty.org/docs/help/gtk-opengl-context

This patch removes the requirement at the GTK level and lets the ghostty
renderer check, now failing as follow:
info(opengl): loaded OpenGL 4.2
error(opengl): OpenGL version is too old. Ghostty requires OpenGL 4.3
warning(gtk_ghostty_surface): failed to initialize surface err=error.OpenGLOutdated
warning(gtk_ghostty_surface): surface failed to initialize err=error.SurfaceError

(Note that this does not render a ghostty window, unlike the previous
error which rendered the "Unable to acquire an OpenGL context for
rendering." view, so while the error itself is easier to understand it
might be harder to view)

src/apprt/gtk/class/surface.zig

@@ -1658,13 +1658,7 @@ pub const Surface = extern struct {
         };
         priv.drop_target.setGtypes(&drop_target_types, drop_target_types.len);
 
-        // Initialize our GLArea. We only set the values we can't set
-        // in our blueprint file.
-        const gl_area = priv.gl_area;
-        gl_area.setRequiredVersion(
-            renderer.OpenGL.MIN_VERSION_MAJOR,
-            renderer.OpenGL.MIN_VERSION_MINOR,
-        );
+        // Setup properties we can't set from our Blueprint file.
         self.as(gtk.Widget).setCursorFromName("text");
 
         // Initialize our config

src/renderer/OpenGL.zig

@@ -137,15 +137,7 @@ fn prepareContext(getProcAddress: anytype) !void {
     errdefer gl.glad.unload();
     log.info("loaded OpenGL {}.{}", .{ major, minor });
 
-    // Enable debug output for the context.
-    try gl.enable(gl.c.GL_DEBUG_OUTPUT);
-
-    // Register our debug message callback with the OpenGL context.
-    gl.glad.context.DebugMessageCallback.?(glDebugMessageCallback, null);
-
-    // Enable SRGB framebuffer for linear blending support.
-    try gl.enable(gl.c.GL_FRAMEBUFFER_SRGB);
-
+    // Need to check version before trying to enable it
     if (major < MIN_VERSION_MAJOR or
         (major == MIN_VERSION_MAJOR and minor < MIN_VERSION_MINOR))
     {
@@ -155,6 +147,15 @@ fn prepareContext(getProcAddress: anytype) !void {
         );
         return error.OpenGLOutdated;
     }
+
+    // Enable debug output for the context.
+    try gl.enable(gl.c.GL_DEBUG_OUTPUT);
+
+    // Register our debug message callback with the OpenGL context.
+    gl.glad.context.DebugMessageCallback.?(glDebugMessageCallback, null);
+
+    // Enable SRGB framebuffer for linear blending support.
+    try gl.enable(gl.c.GL_FRAMEBUFFER_SRGB);
 }
 
 /// This is called early right after surface creation.