[optimize & bug fix] Button: 优化layer, 修复滚动时, gpu占用过高情况 (#336)

* 优化button的layer

* [optimize] Button: 把z-index:0; 位置调整

packages/vant-css/src/button.css

@@ -24,7 +24,7 @@
     border-color: $black;
     background-color: $black;
     border-radius: inherit;/* inherit parent's border radius */
-    transform: translate3d(-50%, -50%, 0); 
+    transform: translate(-50%, -50%); 
   }
 
   &:not([disabled]):active::before {

packages/vant-css/src/loading.css

@@ -12,6 +12,8 @@
 .van-loading {
   font-size: 0;
   line-height: 0;
+  z-index: 0;
+  position: relative;
 
   &--circle {
     width: 16px;
@@ -28,6 +30,8 @@
   &__spinner {
     width: 100%;
     height: 100%;
+    position: relative;
+    z-index: -1;
     display: inline-block;
     animation: van-loading 0.8s linear infinite;