Add half-float vertex attribute caps bit and benchmark

Updates the existing vertex color bench with a half-float mode.
Vanilla GL2/ES2 don't have half-float attributes. The extension
(OES_vertex_half_float) uses a different enum value for the type
parameter. For now, just support GL3/ES3. Otherwise, we could
add a check in GrGLGpu::setupGeometry to re-map GL_HALF_FLOAT
to GL_HALF_FLOAT_OES, based on caps?

Bug: skia:
Change-Id: I1325087db4e615023cceffe1c5b051232769d0ac
Reviewed-on: https://skia-review.googlesource.com/154822
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/gpu/vk/GrVkCaps.cpp b/src/gpu/vk/GrVkCaps.cpp
index 4445a3a..27c2937 100644
--- a/src/gpu/vk/GrVkCaps.cpp
+++ b/src/gpu/vk/GrVkCaps.cpp
@@ -35,6 +35,7 @@
     fBlacklistCoverageCounting = true; // blacklisting ccpr until we work through a few issues.
     fFenceSyncSupport = true;   // always available in Vulkan
     fCrossContextTextureSupport = true;
+    fHalfFloatVertexAttributeSupport = true;
 
     fMapBufferFlags = kNone_MapFlags; //TODO: figure this out
     fBufferMapThreshold = SK_MaxS32;  //TODO: figure this out