glretrace: Use glStringi for all 3.x contexts.

And not just those we explicitly request to be core context.

Fixes issue #236.
gltrace: Use _glGetInteger where possible.

Simplifies/condenses code somewhat.
gltrace: Handle GL_EXT_compiled_vertex_array correctly.

Quake3 puts some vertices in its array buffer that it never renders.
This happens for clouds rendering as well as flares. (In other words, it
creates a vertex buffer with V vertices, and an index buffer that
reference indices substantially lower than V, max(I) < V).

It uses compiled vertex arrays, and will LockArrays(V) - more elements
than it actually references. This is legal because Quake3's arrays have
a static size, much bigger than however many vertices it puts in there,
so the driver can safely read the whole locked area even though half of
it is never referenced.

With Apitrace, however, the situation changes a bit, because Apitrace's
calculation for the length of a user pointer is based on max(draw
element indices).

Below is an example of Quake3's sky rendering:

  3466977 glLockArraysEXT(first = 0, count = 110)
  3466978 glEnableClientState(array = GL_TEXTURE_COORD_ARRAY)
  3466979 glEnableClientState(array = GL_COLOR_ARRAY)
  3466980 glEnableClientState(array = GL_COLOR_ARRAY)
  3466981 glBindTexture(target = GL_TEXTURE_2D, texture = 1132)
  3466982 glTexCoordPointer(size = 2, type = GL_FLOAT, stride = 0, pointer = blob(440))
  3466983 glClientActiveTexture(texture = GL_TEXTURE1)
  3466984 glTexCoordPointer(size = 2, type = GL_FLOAT, stride = 0, pointer = blob(440))
  3466985 glClientActiveTexture(texture = GL_TEXTURE0)
  3466986 glColorPointer(size = 4, type = GL_UNSIGNED_BYTE, stride = 0, pointer = blob(220))
  3466987 glVertexPointer(size = 3, type = GL_FLOAT, stride = 16, pointer = blob(876))
  3466988 glDrawElements(mode = GL_TRIANGLES, count = 192, type = GL_UNSIGNED_INT, indices = blob(768))

The glDrawElements only references indices from 0 to 54.

Under Apitrace, the trace only copies the 55 vertices, instead of the
110 locked values. When replaying, the driver, which may want to read
the whole locked area, will read past the 55 vertices. This doesn't
always crash because we're lucky, but on the NVIDIA driver this crashes
fairly reliably. According to the compiled array spec, the driver is
allowed to read all of the locked memory.

The fix is to change the array tracing so that its count is based on
max(drawelements_count, locked_count).

Thanks to Arthur Huillet for diagnosing the problem, the above
description, suggesting the fix, and testing it.

Tested-by: Arthur Huillet <arthur.huillet@free.fr>
glretrace: avoid glXSwapBuffers failures because of "bad" windows

Both Mesa's and NVIDIA's libGL generate an X protocol error in
glXSwapBuffers() if the window has never been bound with
glXMakeCurrent().  That causes glretrace to exit.  The X protocol
error can't be trapped with XSetErrorHandler() for some reason.
So, keep track of whether a window has ever been bound and skip
glXSwapBuffers if the window has never been bound.

This allows some WGL-based apitraces to be replayed on Linux/GLX.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>