texture-pixmap-x11: initialize ->use_winsys_texture
texture-pixmap-x11: initialize ->use_winsys_texture

This ensures that tex_pixmap->use_winsys_texture is always initialized
during cogl_texture_pixmap_x11_new - either according to the result of
winsys->texture_pixmap_x11_create, or if the winsys doesn't support tfp
we explicitly initialize to FALSE.

Reviewed-by: Neil Roberts <822ec91a7ef82248fd36162ca8ac3283eba70848@linux.intel.com>

gcampax/cogl,djdeath/cogl,collects/cogl,Distrotech/cogl,djdeath/cogl-android,djdeath/cogl,gcampax/cogl,djdeath/cogl-android,spatulasnout/cogl,Distrotech/cogl,djdeath/cogl,Distrotech/cogl,Distrotech/cogl,gcampax/cogl,djdeath/cogl-android,gcampax/cogl,collects/cogl,spatulasnout/cogl