Bump is:pano threshold to 2.0.

Previous value of 1.6 was too small because iPhone 5's aspect ratio
is 1.78, meaning all landscape iPhone 5 photos were included in
is:pano. Same with Moto-X and probably other newer phones.

I tried 1.8, but it still "felt" too small. I think the correct
value is atleast 2.0, but perhaps larger. I don't have enough test
data in that range right now to get a good feel for it.

Change-Id: I6a3e054290d6f434afac8142a9abc68e53008229

pkg/search/expr.go

@@ -158,7 +158,7 @@ func parseExpression(ctx *context.Context, exp string) (*SearchQuery, error) {
 			continue
 		}
 		if word == "is:pano" {
-			andWHRatio(&FloatConstraint{Min: 1.6})
+			andWHRatio(&FloatConstraint{Min: 2.0})
 			continue
 		}
 		if word == "has:location" {

pkg/search/expr_test.go

@@ -62,7 +62,7 @@ var parseExprTests = []struct {
 								File: &FileConstraint{
 									IsImage: true,
 									WHRatio: &FloatConstraint{
-										Min: 1.6,
+										Min: 2.0,
 									},
 								},
 							},