gtk/imgui_widget: remove direct call to glClearColor

Since the refactor to move our OpenGL context off-thread there is no more
GLAD context loaded on the main thread for this widget, so calling ANY
OpenGL function will crash the entire app.

I don't think the call even worked as intended? I at least can't seem to
tell any difference when the clear commands are simply removed.
Maybe they were useful before.

passthrough.glsl

@@ -0,0 +1,4 @@
+void mainImage(out vec4 fragColor, in vec2 fragCoord) {
+    vec2 uv = fragCoord / iResolution.xy;
+    fragColor = vec4(texture(iChannel0, uv).rgb, 1.0);
+}

src/apprt/gtk/class/imgui_widget.zig

@@ -317,9 +317,6 @@ pub const ImguiWidget = extern struct {
             cimgui.c.ImGui_Render();
         }
 
-        // OpenGL final render
-        gl.clearColor(0x28 / 0xFF, 0x2C / 0xFF, 0x34 / 0xFF, 1.0);
-        gl.clear(gl.c.GL_COLOR_BUFFER_BIT);
         cimgui.ImGui_ImplOpenGL3_RenderDrawData(cimgui.c.ImGui_GetDrawData());
 
         return @intFromBool(true);