devdraw: cocoa metal screen updates (#215)

* devdraw: cocoa metal screen uses a dirty hack to make everything smooth

* devdraw: cocoa metal screen uses a layer to make fullscreen applications behave

* devdraw: macOS cocoa metal fix resizeimg without img

* devdraw: macOS cocoa metal uses blit instead of render

We directly use the blit command encoder to copy texture to the
framebuffer.  We no longer need to compile the metal shader every
time the application starts just for rendering a flat 2D surface.

* travis: add osx images covering 10.13 and 10.14

.travis.yml

@@ -2,6 +2,10 @@ language: c
 
 matrix:
   include:
+    - os: osx
+      osx_image: xcode10.2
+    - os: osx
+      osx_image: xcode9.4
     - os: osx
       osx_image: xcode9
     - os: osx