Allow the default background color to be changed

[robert-ancell]

Fixes flutter/flutter#149920

[robert-ancell]

To enable this is an application edit my_application.cc and add code like:

GdkRGBA background_color;
gdk_rgba_parse(&background_color, "#ff0000");
fl_view_set_background_color(view, &background_color);

[robert-ancell]

This doesn't solve all the issues with background, but might be useful for those who want to immediately minimize the black background shown when an application starts.

[robert-ancell]

@loic-sharma this might also affect the Windows embedder - it seems like you should be allowed to set the background for an app (e.g. red) and Flutter can have a transparent background (e.g. with scaffoldBackgroundColor: Colors.transparent in ThemeData). In that case we can't use glBlitFramebuffer to render this correctly and must use textures and shaders to do this. However, this is probably not the common case for Flutter apps, is it possible for Flutter to indicate if any content has alpha? The other case is presenting multiple layers - I'm currently blitting those but in general should they also be assumed to contain alpha content?

[cbracken]

[loic-sharma]

LGTM

[loic-sharma]

@loic-sharma this might also affect the Windows embedder - it seems like you should be allowed to set the background for an app (e.g. red) and Flutter can have a transparent background (e.g. with scaffoldBackgroundColor: Colors.transparent in ThemeData). In that case we can't use glBlitFramebuffer to render this correctly and must use textures and shaders to do this. However, this is probably not the common case for Flutter apps, is it possible for Flutter to indicate if any content has alpha? The other case is presenting multiple layers - I'm currently blitting those but in general should they also be assumed to contain alpha content?

Agreed. We'll definitely need to fix this before we can support platform views on Windows.

[robert-ancell]

The bug with the background not showing through is a bug in GTK version in Ubuntu, which is fixed upstream. So I've re-enabled the transparency in this change.

[robert-ancell]

Interestingly when using a fixed version of GTK, the blit does seem to work with alpha. I'm not sure if that's correct OpenGL behaviour or driver specific.

[robert-ancell]

Talking to another developer the Blit alpha behaviour might be Mesa specific, so when testing on NVIDIA it might not work.