Fix swizzles in clear_swizzle GM

Original version swizzled the alpha value. This isn't really allowed
because of blend equations that use alpha coeffs. When doing
color+alpha drawing we have to use the format's alpha channel
as alpha in the swizzle.

TBR: michaelludwig@google.com

Bug: skia:11019
Change-Id: I38922b966d37974b250904ad5c0ea825728b8c77
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/340340
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

Author: bsalomon

gm/clear_swizzle.cpp

@@ -21,8 +21,8 @@ DEF_SIMPLE_GPU_GM(clear_swizzle, ctx, rtCtx, canvas, 6*kSize, 2*kSize) {
     }
 
     auto make_offscreen = [&](const SkISize dimensions) {
-        GrSwizzle readSwizzle  = GrSwizzle::Concat(rtCtx->readSwizzle(), GrSwizzle{"garb"});
-        GrSwizzle writeSwizzle = GrSwizzle::Concat(rtCtx->readSwizzle(), GrSwizzle{"brag"});
+        GrSwizzle readSwizzle  = GrSwizzle::Concat(rtCtx->readSwizzle(), GrSwizzle{"bgra"});
+        GrSwizzle writeSwizzle = GrSwizzle::Concat(rtCtx->readSwizzle(), GrSwizzle{"bgra"});
         return GrRenderTargetContext::Make(ctx,
                                            rtCtx->colorInfo().refColorSpace(),
                                            SkBackingFit::kExact,