[qtdeclarative] Ensure that ShaderEffectSource's texture has the right parameter.

The provider is created on demand in textureProvider(), but its values
are populated in updatePaintNode() so if updatePaintNode() was called
before a consumer asked for the texture provider, the texture parameters
would be wrong.

Change-Id: I34b42976c4f852557400b1886978f1dc36a0a7a8