Query extensions at runtime

Don't run extensions unless they're available. This includes querying
for descriptor indexing, and running one of two versions of kernel4
depending on whether it's enabled.

Part of the support needed for #78

Author: raphlinus

Cargo.lock

@@ -157,7 +157,12 @@ fn gen_enum_read(
         writeln!(r, "{}Tag {}_tag({}Ref ref) {{", name, name, name).unwrap();
         writeln!(r, "    uint tag_and_flags = {}[ref.offset >> 2];", bufname).unwrap();
     }
-    writeln!(r, "    return {}Tag(tag_and_flags & 0xffff, tag_and_flags >> 16);", name).unwrap();
+    writeln!(
+        r,
+        "    return {}Tag(tag_and_flags & 0xffff, tag_and_flags >> 16);",
+        name
+    )
+    .unwrap();
     writeln!(r, "}}\n").unwrap();
     for (var_name, payload) in variants {
         let payload_ix = if payload.len() == 1 {
@@ -564,7 +569,9 @@ fn gen_enum_write(
                 }
                 writeln!(r, "}}\n").unwrap();
             }
-        } else if payload.len() == 2 && matches!(payload[0].1.ty, GpuType::Scalar(GpuScalar::TagFlags)) {
+        } else if payload.len() == 2
+            && matches!(payload[0].1.ty, GpuType::Scalar(GpuScalar::TagFlags))
+        {
             if let GpuType::InlineStruct(structname) = &payload[1].1.ty {
                 if is_mem {
                     writeln!(

piet-gpu-derive/src/glsl.rs

@@ -8,6 +8,5 @@ edition = "2018"
 
 [dependencies]
 ash = "0.31"
-once_cell = "1.3.1"
 ash-window = "0.5"
 raw-window-handle = "0.3"

piet-gpu-hal/Cargo.toml

@@ -209,6 +209,13 @@ impl Session {
     pub unsafe fn create_sampler(&self, params: SamplerParams) -> Result<Sampler, Error> {
         self.0.device.create_sampler(params)
     }
+
+    /// Report whether the device supports descriptor indexing.
+    ///
+    /// As we have more queries, we might replace this with a capabilities structure.
+    pub fn has_descriptor_indexing(&self) -> bool {
+        self.0.device.has_descriptor_indexing
+    }
 }
 
 impl CmdBuf {