Shader: don't update the shader key from within vrend_convert_shader
As described in #132 (closed) updating the shader key from within vrend_convert_shader
will result in recreating the shader with each call to vrend_select_shader
, so don't update the shader key after it has been created and used in the comparison to check for shader variants.
Fixes #132 (closed) (with !293 (merged) applied too)