Do not allow setting too small or big scale factors

Add a check to constraint the allowed device scale factors to the
[0.05, 5.0] range, ignoring values outside the interval. For debug
builds, an assertion will also make it easier for developers to fix
bugs in code that calls the function with invalid values.

Setting the device scale factor to unreasonably low values can result in
divisions by zero, wrong rendering, and/or odd behaviour in general;
while for too big values graphics buffers will consume unreasonably big
amounts of memory or the maximum usable size for them will result in
failure to allocate buffers and content being partially rendered (if at
all).

Fixes #89

Author: aperezdc

src/view-backend.c

@@ -170,6 +170,11 @@ wpe_view_backend_dispatch_get_accessible(struct wpe_view_backend* backend)
 void
 wpe_view_backend_dispatch_set_device_scale_factor(struct wpe_view_backend* backend, float scale)
 {
+    if (scale < 0.05f || scale > 5.0f) {
+        assert(!"Scale factor not in the [0.05, 5.0] range");
+        return;
+    }
+
     if (backend->backend_client && backend->backend_client->set_device_scale_factor)
         backend->backend_client->set_device_scale_factor(backend->backend_client_data, scale);
 }