Keep the color target as an FP16 format in hgiInterop

Keep the colour target as an FP16 in HgiInteropMetal, which allows openGL clients to use EDR.  This doesn't change the behaviour in usdview, we'll have to add some changes into the Python code to create the right final target.

Contribution: Apple

Fixes #2482

(Internal change: 2281903)

pxr/imaging/hgiInterop/metal.mm

@@ -549,7 +549,7 @@
         kCFAllocatorDefault,
         width,
         height,
-        kCVPixelFormatType_32BGRA,
+        kCVPixelFormatType_64RGBAHalf,
         (__bridge CFDictionaryRef)cvBufferProperties,
         &_pixelBuffer);
 
@@ -598,7 +598,7 @@
         _cvmtlTextureCache,
         _pixelBuffer,
         (__bridge CFDictionaryRef)metalTextureProperties,
-        MTLPixelFormatBGRA8Unorm,
+        MTLPixelFormatRGBA16Float,
         width,
         height,
         0,