Query the framebuffer stencil bits instead of assuming it's global

Previously when the context was initialised Cogl would query the
number of stencil bits and set a private feature flag to mark that it
can use the buffer for clipping if there was at least 3. The problem
with this is that the number of stencil bits returned by
GL_STENCIL_BITS depends on the currently bound framebuffer. This patch
adds an internal function to query the number of stencil bits in a
framebuffer and makes it use that instead when determining whether it
can push the clip using the stencil buffer.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

(cherry picked from commit e928d21516a6c07798655341f4f0f8e3c1d1686c)

cogl/cogl-framebuffer.c

@@ -1144,6 +1144,17 @@ cogl_framebuffer_get_depth_bits (CoglFramebuffer *framebuffer)
   return bits.depth;
 }
 
+int
+_cogl_framebuffer_get_stencil_bits (CoglFramebuffer *framebuffer)
+{
+  CoglContext *ctx = framebuffer->context;
+  CoglFramebufferBits bits;
+
+  ctx->driver_vtable->framebuffer_query_bits (framebuffer, &bits);
+
+  return bits.stencil;
+}
+
 CoglColorMask
 cogl_framebuffer_get_color_mask (CoglFramebuffer *framebuffer)
 {