Fix web crash on missing uniform buffer padding

The alignment is actually already there, but wgpu/Naga doesn't see it at this point.

crates/re_renderer/shader/lines.wgsl

@@ -13,6 +13,10 @@ var position_data_texture: texture_2d<u32>;
 
 struct DrawDataUniformBuffer {
     size_boost_in_points: f32,
+    // In actuality there is way more padding than this since we size all uniform buffers are 256bytes aligned in order to be sub-allocatable.
+    // But wgpu doesn't know this and therefore requires `DownlevelFlags::BUFFER_BINDINGS_NOT_16_BYTE_ALIGNED` if we wouldn't add padding here,
+    // which isn't available on WebGL.
+    _padding: Vec4,
 };
 @group(1) @binding(2)
 var<uniform> draw_data: DrawDataUniformBuffer;

crates/re_renderer/shader/point_cloud.wgsl

@@ -12,6 +12,10 @@ var color_texture: texture_2d<f32>;
 
 struct DrawDataUniformBuffer {
     size_boost_in_points: f32,
+    // In actuality there is way more padding than this since we size all uniform buffers are 256bytes aligned in order to be sub-allocatable.
+    // But wgpu doesn't know this and therefore requires `DownlevelFlags::BUFFER_BINDINGS_NOT_16_BYTE_ALIGNED` if we wouldn't add padding here,
+    // which isn't available on WebGL.
+    _padding: Vec4,
 };
 @group(1) @binding(2)
 var<uniform> draw_data: DrawDataUniformBuffer;