windows opengl: Explain the concept of the text drawing + minor fixes.

Change-Id: I4651e7e9b8163844be548d4ed975a881d4f83fff
windows opengl: Finally got the text working.

It still does not have a transparent background, but that is pending support in
OpenGLSalGraphicsImpl.

Change-Id: I477a483e6ac940f54f6ffd6816d753d87206bf23
vcl: re-factor image rendering control-flow.

Change-Id: I923f92e5e84d35dafd1c0d1b4d63916ce690cb90
vcldemo: much improved icon rendering tests.

Change-Id: I853bc0a0957cf0db629a0a5f1ced67bc069d8403
vcl: Fix palette modification during BitmapWriteAccess lifetime.

Change-Id: I8bdedd63895ff0b3245d996cf35ac92d9ab0ff9d
vcldemo: dump and render all icons from images.zip when zoomed in.

Change-Id: I89f37b8a1ed70334a3485bc3ca06d04cfe6d0827