glsl: struct constructors/initializers only allow implicit conversions

glsl: struct constructors/initializers only allow implicit conversions

When an argument for a structure constructor or initializer doesn't
match the expected type, only Section 4.1.10 “Implicit Conversions”
are allowed to try to match that expected type.

From page 32 (page 38 of the PDF) of the GLSL 1.20 spec:

  " The arguments to the constructor will be used to set the structure's
    fields, in order, using one argument per field. Each argument must
    be the same type as the field it sets, or be a type that can be
    converted to the field's type according to Section 4.1.10 “Implicit
    Conversions.”"

From page 35 (page 41 of the PDF) of the GLSL 4.20 spec:

  " In all cases, the innermost initializer (i.e., not a list of
    initializers enclosed in curly braces) applied to an object must
    have the same type as the object being initialized or be a type that
    can be converted to the object's type according to section 4.1.10
    "Implicit Conversions". In the latter case, an implicit conversion
    will be done on the initializer before the assignment is done."

v2: Remove also the now redundant constant conversion, the
    constant_record_constructor helper and the replacement code
    (Timothy).

Fixes GL44-CTS.shading_language_420pack.initializer_list_negative

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>