pipeline: Avoid reseting texture target for NULL textures
pipeline: Avoid reseting texture target for NULL textures

When setting a NULL texture on a CoglPipeline we would also reset the
texture target to a dummy value of 0. Reseting the target also had the
effect of making fragends discard any associated program. In cases where
the NULL texture was only transient until a replacement texture could be
set we were re-running lots of redundant codegen and shader
compilations.

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

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