I found that the gears demo was spending 40% cpu
downloading a GL texture every frame, only to
upload it again to another context.

While the GSK rendering and the GtkGLArea use different
GL contexts, they are (usually) connected by sharing data
with the same global context, so we can just use the
texture without the download/upload dance. This brings
gears down to < 10% cpu.

GtkTreeviewColumn: don't focus-on-click header buttons

See merge request GNOME/gtk!1806

Copy the format conversion code from GdkMemoryTexture
so we can produce all formats, and test them all.

The upload fast paths assume that the stride is a
multiple of four, so some of the padding values cause
it to fail. Apart from that, things seem to work for
all combinations.

Matthiasc/for master

See merge request GNOME/gtk!2626

Create textures with various characteristics (alpha, premultiplication,
stride) that trigger different code paths in the gl texture upload
function, and show the resulting images. If all goes well, they all
should look the same.

On my system, this tests texture upload for memory formats
GDK_MEMORY_B8G8R8A8_PREMULTIPLIED, GDK_MEMORY_R8G8B8A8, and
GDK_MEMORY_R8G8B8, and it works with both gl and gles.

More texture upload fixes

Closes #3198

See merge request GNOME/gtk!2624

gl: Make debug spew more complete

Closes #3197

See merge request GNOME/gtk!2623

In some cases, with bpp == 3 and a rowstride
that is divisible by 4, we were passing invalid
parameters to GL.

Fixes: #3198

Make GtkBuildable vfunc accessor functions private

Closes #3191

See merge request GNOME/gtk!2619