More fixes for building with vs2019/msvc (#1500)

* Update Froxelizer.h

Fix this error when building with msvc from vs2019
error C2926:  'filament::details::Froxelizer::FroxelEntry::<unnamed-tag>::offset': a default member initializer is not allowed for a member of an anonymous struct within a union

* Fix some compilation issues with vs2019/msvc

Program.cpp:
1>C:\greg\github\filament\filament\backend\src\Program.cpp(28,42): error C2610:  'filament::backend::Program::Program(void) noexcept': is not a special member function or comparison operator which can be defaulted
1>C:\greg\github\filament\filament\backend\src\Program.cpp(28,42): message :  exception specification does not match the implicitly declared specification.

GLUtils.h: __PRETTY_FUNCTION__ macro is clang specific. Use MSVC equivalent

Color.h: fix warning

* #1493 - inline constructor in definition as requested by @romainguy

* #1493  "move this #define inside the #else below" as requested

* #1493 revert last change which causes compilation failures on other platforms.

provide empty implementation of Program::Program() in Program.cpp

* More fixes for building with vs2019/msvc

* #1500 use consistent macro definition syntax (@bejado)

* #1500 simplify DEBUG_COMMAND macro as requested by @pixelflinger

* #1500 use `{ 0 }` which is accepted by Visual Studio  (` = 0 ` is not accepted)

* #1500 remove incorrect UTILS_RESTRICT alltogether

libs/math/dummy.cpp

@@ -15,6 +15,8 @@
  */
 
 // Empty symbol used to suppress warnings on Darwin
-__attribute__((visibility("hidden")))
+#if __has_attribute(visibility) 
+    __attribute__((visibility("hidden")))
+#endif
 void e() {
 }