gpu/shaders: avoid first-class arrays

Safari's WebGL1 implementation (rightly) complains that first-class
array types are not supported as function result types. Define and
use a struct type instead.

While we're here, use const variables instead of functions.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/shaders/cover.vert

@@ -23,7 +23,6 @@ layout(location = 1) out vec2 vUV;
 void main() {
     gl_Position = toClipSpace(vec4(pos*transform.xy + transform.zw, z, 1));
 	vUV = uv*uvTransform.xy + uvTransform.zw;
-	vec3[2] fboTrans = fboTextureTransform();
-	vec3 uv3 = transform3x2(fboTrans, vec3(uv, 1.0));
+	vec3 uv3 = transform3x2(fboTextureTransform, vec3(uv, 1.0));
 	vCoverUV = (uv3*vec3(uvCoverTransform.xy, 1.0)+vec3(uvCoverTransform.zw, 0.0)).xy;
 }