shaders: Make sure the shader key is not updated from within vrend_convert_create

The shader key used to create a shader variant is stored alongside with the shader and will be used to search for the variant by comparing to a key with values filled in beforehand. If the shader key is updated from within vrend_convert_shader then it will never match a shader key that is created beforehand, because it uses additional information not taken into account when the search key is evaluated. As a result new shader variants will be created whenever the shader is looked up, and memory usage will grow limitless.