qhighdpiscaling: de-pessimize qEnvironmentVariableOptional*() functions

Don't call qEnvironmentVariableIsSet(), since it locks the environment
mutex. Since qgetenv() and qEnvironmentVariable() both report whether
the value was set (returning non-isNull()), we can avoid the extra
mutex lock (and envvar lookup) by just calling those and then checking
for isNull().

In qEnvironmentVariableOptionalReal(), in addition, use qgetenv()
instead of qEnvironmentVariable(). While on Windows, that may convert
encoding, QString::toDouble() first converts to Latin-1, and only then
converts to double, so we avoid one encoding conversion, even on
Windows; on Unix we avoid two.

Amends 4d1f13f.

Pick-to: 6.9 6.8 6.5
Change-Id: I6803e2277b324c2568726765245681bcbf517362
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>

src/gui/kernel/qhighdpiscaling.cpp

@@ -30,18 +30,14 @@ static const char usePhysicalDpiEnvVar[] = "QT_USE_PHYSICAL_DPI";
 
 static std::optional<QString> qEnvironmentVariableOptionalString(const char *name)
 {
-    if (!qEnvironmentVariableIsSet(name))
-        return std::nullopt;
-
-    return std::optional(qEnvironmentVariable(name));
+    QString value = qEnvironmentVariable(name);
+    return value.isNull() ? std::nullopt : std::optional(std::move(value));
 }
 
 static std::optional<QByteArray> qEnvironmentVariableOptionalByteArray(const char *name)
 {
-    if (!qEnvironmentVariableIsSet(name))
-        return std::nullopt;
-
-    return std::optional(qgetenv(name));
+    QByteArray value = qgetenv(name);
+    return value.isNull() ? std::nullopt : std::optional(std::move(value));
 }
 
 static std::optional<int> qEnvironmentVariableOptionalInt(const char *name)
@@ -54,11 +50,12 @@ static std::optional<int> qEnvironmentVariableOptionalInt(const char *name)
 
 static std::optional<qreal> qEnvironmentVariableOptionalReal(const char *name)
 {
-    if (!qEnvironmentVariableIsSet(name))
+    const QByteArray val = qgetenv(name);
+    if (val.isNull())
         return std::nullopt;
 
     bool ok = false;
-    const qreal value = qEnvironmentVariable(name).toDouble(&ok);
+    const qreal value = val.toDouble(&ok);
     return ok ? std::optional(value) : std::nullopt;
 }