Fill in some missing bits of capability API (#1652)

* Fill in some missing bits of capability API

* Make invalid/unknown capability have a zero value (this aligns it better with the public API for `SlangProfileID`, so that the two can be merged down the line)

* Actually provide an implementation of `spFindCapability()` public API

* fixup: bug fixes for renumbering invalid capability atom

1 files changed, 4 insertions, 0 deletions

diff --git a/slang.h b/slang.h
index a0487303b..6c9ab3eea 100644
--- a/slang.h
+++ b/slang.h
@@ -642,6 +642,10 @@ extern "C"
     };
 
     typedef SlangInt32 SlangCapabilityID;
+    enum
+    {
+        SLANG_CAPABILITY_UNKNOWN = 0,
+    };
 
     typedef unsigned int SlangMatrixLayoutMode;
     enum