Deal with not being able to require 'Cairo' better

Previous the 'gtk_simple_use_cairo' would attempt to require Cairo
and would fail silently.  In turn the 'add-draw-handler' would then
attempt to to use values derived from the module by type lookup amd
pass them to a native routine, resulting in an exception in the
native code and subsequently a confusing MoarVM panic when they are
undefined.

This takes a belt and braces approach adding a Bool return to the
gtk_simple_use_cairo so user code can check and act appropriately
if Cairo can't be loaded, and the add-draw-handler will also
throw an exception if called without the module loaded.

Fixes #85

examples/18-cairo-draw-handler.pl6

@@ -10,20 +10,26 @@ use GTK::Simple::DrawingArea;
 
 my $app = GTK::Simple::App.new(:title<Test>);
 my $da = GTK::Simple::DrawingArea.new;
-gtk_simple_use_cairo;
-
-$app.set-content( $da );
-$app.border-width = 20;
-$da.size-request(300,300);
-
-sub rect-do( $d, $ctx ) {
-    given $ctx {
-	.rgb(0, 0.7, 0.9);
-	.rectangle(10, 10, 50, 50);
-	.fill :preserve; .rgb(1, 1, 1);
-	.stroke
-    };
+
+if gtk_simple_use_cairo()  {
+
+    $app.set-content( $da );
+    $app.border-width = 20;
+    $da.size-request(300,300);
+
+    sub rect-do( $d, $ctx ) {
+        given $ctx {
+        .rgb(0, 0.7, 0.9);
+        .rectangle(10, 10, 50, 50);
+        .fill :preserve; .rgb(1, 1, 1);
+        .stroke
+        };
+    }
+
+    my $ctx = $da.add-draw-handler( &rect-do );
+    $app.run;
 }
+else {
+    note "'Cairo' module is not installed - can't run example";
 
-my $ctx = $da.add-draw-handler( &rect-do );
-$app.run;
+}

lib/GTK/Simple/DrawingArea.pm6

@@ -10,13 +10,19 @@ need GTK::Simple::ConnectionHandler;
 
 my Mu $cairo_t;
 my Mu $Cairo_Context;
+my Bool $cairo-loaded = False;
 
-sub gtk_simple_use_cairo() is export {
-    try {
-        require Cairo;
+sub gtk_simple_use_cairo( --> Bool ) is export {
+    try require ::('Cairo');
+    if ::('Cairo') ~~ Failure {
+        $cairo-loaded = False;
+    }
+    else {
         $cairo_t := ::('Cairo')::('cairo_t');
         $Cairo_Context := ::('Cairo')::('Context');
+        $cairo-loaded = True;
     }
+    $cairo-loaded;
 }
 
 unit class GTK::Simple::DrawingArea does GTK::Simple::Widget;
@@ -26,6 +32,7 @@ submethod BUILD() {
 }
 
 method add-draw-handler(&handler) {
+    die "'Cairo' module is not loaded, check it is installed and that you called 'gtk_simple_use_cairo'" unless $cairo-loaded;
     my sub handler_wrapper($widget, $cairop) {
         my $cairo  = nqp::box_i($cairop.Int, $cairo_t);
         my $ctx    = $Cairo_Context.new($cairo);