Merge #741

741: Fix the default lod_max_clamp value for the SamplerDescriptor r=kvark a=hasenbanck

**Connections**
As suggested in https://github.com/gfx-rs/wgpu-rs/pull/397

**Description**
The current default value for the ```lod_max_clamp``` of a sampler is 0.
This would deactivate mipmapping alltogether. Setting the default to ```f32::MAX``` makes sure to always enable it. It's the default value apple uses in Metal, it seems to be valid usage in Vulkan (```maxLod``` in ```VkSamplerCreateInfo```) and in DX12 (```MaxLOD``` in ```D3D12_SAMPLER_DESC```).

Co-authored-by: Nils Hasenbanck <nils@hasenbanck.de>

wgpu-types/src/lib.rs

@@ -1431,7 +1431,7 @@ impl Default for FilterMode {
 }
 
 /// Describes a [`Sampler`]
-#[derive(Default, Clone, Debug, PartialEq)]
+#[derive(Clone, Debug, PartialEq)]
 #[cfg_attr(feature = "trace", derive(Serialize))]
 #[cfg_attr(feature = "replay", derive(Deserialize))]
 pub struct SamplerDescriptor<L> {
@@ -1464,6 +1464,25 @@ pub struct SamplerDescriptor<L> {
     pub _non_exhaustive: NonExhaustive,
 }
 
+impl<L: Default> Default for SamplerDescriptor<L> {
+    fn default() -> Self {
+        Self {
+            label: Default::default(),
+            address_mode_u: Default::default(),
+            address_mode_v: Default::default(),
+            address_mode_w: Default::default(),
+            mag_filter: Default::default(),
+            min_filter: Default::default(),
+            mipmap_filter: Default::default(),
+            lod_min_clamp: 0.0,
+            lod_max_clamp: f32::MAX,
+            compare: Default::default(),
+            anisotropy_clamp: Default::default(),
+            _non_exhaustive: Default::default(),
+        }
+    }
+}
+
 impl<L> SamplerDescriptor<L> {
     pub fn map_label<K>(&self, fun: impl FnOnce(&L) -> K) -> SamplerDescriptor<K> {
         SamplerDescriptor {