Use downlevel_limits for RPi5 compatibility

Limits::default() requests max_color_attachments=8, but RPi5 Vulkan
only supports 4. downlevel_defaults() uses 4, which is all we need.

src/renderer.rs

@@ -106,7 +106,7 @@ pub fn start(
         &wgpu::DeviceDescriptor {
             label: None,
             required_features: wgpu::Features::PASSTHROUGH_SHADERS,
-            required_limits: wgpu::Limits::default(),
+            required_limits: wgpu::Limits::downlevel_defaults(),
             experimental_features: wgpu::ExperimentalFeatures::disabled(),
             memory_hints: wgpu::MemoryHints::Performance,
             trace: wgpu::Trace::Off,