mesa: rework viewport/scissor initialization code

mesa: rework viewport/scissor initialization code

The first time a context is bound to a drawable, the viewport and scissor
bounds are initialized to the buffer's size.  This is actually a bit tricky.

A new _mesa_check_init_viewport() function is called in several places
to check if the viewport has been initialized.  We also use a new
ctx->ViewportInitialized flag instead of the overloaded
ctx->FirstTimeCurrent flag.