Fall back to the default visual if no X8R8G8B8.
Fall back to the default visual if no X8R8G8B8.

Bug 18510357

Change-Id: I2406106482b8c65408df783ebb78f5abd17a6320
Reviewed-on: https://swiftshader-review.googlesource.com/1593
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>

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