glsl: make minimum struct size = 2, not 1

glsl: make minimum struct size = 2, not 1

1-component structs such as "struct foo { float x; }" could get placed at
any position within a register.  This caused some trouble computing the
field offset which assumed all struct objects were placed at R.x.
It would be unusual to hit this case in normal shaders.

(cherry picked from master, commit ca03e881a8d8fa3e36a601238559c20311373633)