Let's pretend it's 2008 and forget about multisample textures for
a moment. Therefore, add support for color renderbuffers. These are
only useful when sample count is > 1 i.e. multisampling is requested.
In combination with the yet-to-be-added resolve operation this would
allow doing msaa offscreen rendering on GLES 3.0 (or GLES 2.0 with
extensions) systems as well (those do not have multisample textures)

Backends other than gl just use a texture under the hood. With OpenGL
we use a renderbuffer, however, not a texture.

This may seem like a horrible waste of time but will eventually be
helpful to get a port of Qt Quick that is feature-wise identical to
today's OpenGL path, without changing the OpenGL version requirements.

Introduces isFeatureSupported, so examples like msaatexture can
conveniently qFatal (hehe) when the GL backend reports multisample
textures are not supported.

While we are at it, make some examples nicer by not having to repeat
releasing resources one by one.

The example (msaarenderbuffer) is not finished yet since adding support
for resolve (vkCmdResolveImage, glBlitFrameBuffer, etc.) is a future task.