Fix BCn related SwANGLE test failure
Fix BCn related SwANGLE test failure

There was a missing prepareForSampling call after
doing an image copy between 2 compressed textures.

This cl fixes these angle end to end tests:
CopyCompressedTextureTest.Basic/ES3_Vulkan_SwiftShader
CopyCompressedTextureTest.Basic/ES2_Vulkan_SwiftShader

Bug: b/146052572 b/146157683
Change-Id: I8703fa71557509e9ecb0d52a4ad2da4c72fa6812
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39448
Presubmit-Ready: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Tested-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Kokoro-Presubmit: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader