Add support color renderbuffers
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.
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