[enhancement] support proper per-pixel alpha for alpha-enabled display framebuffers

I'm still not convinced using an alpha channel is right for transitions. More likely you just want to modify surface opacity for blending (which is a feature that does not per-pixel alpha).

Daniel,

Yes I agree some transitions can be handled by just surface global alpha; however, I think if we are going to expose an alpha-enabled display framebuffer as we do now then we should properly support it, otherwise we should not be creating display buffers with mir_pixel_format_abgr_8888 or mir_pixel_format_argb_8888.

Alberto:

This is untrue: "expose an alpha-enabled display framebuffer as we do now"

1. Our servers don't have any alpha channel by default:
    EGLint const config_attr[] = {
        EGL_SURFACE_TYPE, EGL_WINDOW_BIT,
        EGL_RED_SIZE, 8,
        EGL_GREEN_SIZE, 8,
        EGL_BLUE_SIZE, 8,
        EGL_ALPHA_SIZE, 0, <==== No alpha requested

2. Our clients (those that are smart enough) don't have alpha by default either:
   float mir_eglapp_background_opacity = 1.0f; ====> results in non-alpha pixel format selection

But those playing at home should remember that you don't need an alpha channel in either the server or client to do alpha-blending. You still get working shadows and Alt+mousewheel opacity in demo-shell without the need for an alpha channel in the destination buffer.

On 1) I see... however it seems that's for the MESA platform only though.

The android platform can and does expose alpha enabled display framebuffers. I'll investigate why.

And yes of course you can do alpha blending without an alpha channel in the destination buffer. But that's not what this enhancement is about at all.

Note that Mir's current blending function, and the one most graphics code will ever use, does not read-back the destination alpha at all:
         glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

Thus the target buffer will never need alpha... unless we add fancy effects to the contrary later.

Fix committed into lp:mir/devel at revision None, scheduled for release in mir, milestone Unknown