winhttp: Move common code to the upper level, avoid accessing the buffer before the buffer size check, make the code conform to existing rules.

Add some comments to make it easier to identify failing tests.

Note that using GL_DEPTH_COMPONENT instead of eg. GL_DEPTH_COMPONENT24
will work, but will create a renderbuffer with the format of the
onscreen depth buffer.

Spotted by Jeremy White.

Avoids special casing all over the place.