optionally initialize a GtkCanvas immediately

This is helpful for precompilation. GtkCanvas was one of the few widgets where being realized was important to calling its functions. With this we can avoid popping up windows during precompilation.

This also changes the behavior of the width and height arguments of the GtkCanvas constructor. We now set the "content area" size rather than the initial size of the widget. I think this is more useful because it sets a minimum size for the widget. The initial size can still be set by calling "size_request" on the widget after construction.

examples/canvas.jl

@@ -1,6 +1,6 @@
 using Gtk4
 
-c = GtkCanvas()
+c = GtkCanvas(100,100)
 
 # define a function that will be called every time the widget needs to be drawn
 @guarded draw(c) do widget

src/GLib/loop.jl

@@ -133,7 +133,7 @@ Does not affect loop operation if GApplication's `run()` method is being used in
 See also [`start_main_loop`](@ref).
 """
 function stop_main_loop(wait=false)
-    if !g_main_running[] # we are either already stopped, or in a GApplication
+    if !g_main_running[] # we are either already stopped or in a GApplication
         return
     end
     g_main_running[] = false

src/cairo.jl

@@ -10,6 +10,22 @@ function canvas_draw_backing_store(w, cr, width, height, user_data) # cr is a Ca
     ccall((:cairo_paint, libcairo), Nothing, (Ptr{Nothing},), cr)
 end
 
+function _init_canvas!(widget, w, h)
+    widget.back = CairoARGBSurface(w, h)
+    widget.backcc = CairoContext(widget.back)
+end
+
+"""
+    GtkCanvas(w = -1, h = -1, init_back = false; kwargs...)
+
+Create a GtkCanvas widget for drawing using Cairo (based on GtkDrawingArea).
+Optional arguments `w` and `h` can be used to set the minimum width and height
+of the drawing area in pixels. If `init_back` is set to true, the canvas's
+image CairoSurface will be initialized immediately, which is useful for
+precompilation.
+
+Keyword arguments can be used to set properties of the GtkDrawingArea widget.
+"""
 mutable struct GtkCanvas <: GtkDrawingArea # NOT a GType
     handle::Ptr{GObject}
     is_sized::Bool
@@ -18,10 +34,18 @@ mutable struct GtkCanvas <: GtkDrawingArea # NOT a GType
     back::CairoSurface   # backing store
     backcc::CairoContext
 
-    function GtkCanvas(w = -1, h = -1; kwargs...)
+    function GtkCanvas(w = -1, h = -1, init_back = false; kwargs...)
         da = GtkDrawingArea(; kwargs...)
-        G_.set_size_request(da, w, h)
+        if w > 0 && h > 0
+            G_.set_content_height(da, h)
+            G_.set_content_width(da, w)
+        elseif init_back
+            error("Width and height arguments must be provided to immediately initialize GtkCanvas.")
+        end
         widget = new(getfield(da,:handle), false, nothing, nothing)
+        if init_back
+            _init_canvas!(widget, w, h)
+        end
 
         function on_realize(da::GtkWidget)
             if widget.is_sized
@@ -34,8 +58,7 @@ mutable struct GtkCanvas <: GtkDrawingArea # NOT a GType
         function on_resize(da::GtkDrawingArea, width::Cint, height::Cint)
             widget.is_sized = true
             if G_.get_realized(widget)
-                widget.back = CairoARGBSurface(width, height)
-                widget.backcc = CairoContext(widget.back)
+                _init_canvas!(widget, width, height)
 
                 if isa(widget.resize, Function)
                     widget.resize(widget)
@@ -83,13 +106,23 @@ function draw(widget::GtkCanvas)
     G_.queue_draw(widget)
 end
 
+"""
+    getgc(c::GtkCanvas)
+
+Return the CairoContext of the CairoSurface for a GtkCanvas.
+"""
 function getgc(c::GtkCanvas)
     if !isdefined(c,:backcc)
       error("GtkCanvas not yet initialized.")
     end
     return c.backcc
 end
 
+"""
+    cairo_surface(c::GtkCanvas)
+
+Return the image CairoSurface for a GtkCanvas.
+"""
 function cairo_surface(c::GtkCanvas)
     if !isdefined(c,:back)
       error("GtkCanvas not yet initialized.")