add layer c++ interface setAlpha to ios and android

Author: leiyue123

android/libpag/src/main/java/org/libpag/PAGLayer.java

@@ -172,6 +172,12 @@ public Matrix getTotalMatrix() {
      */
     public native void setExcludedFromTimeline(boolean value);
 
+    /**
+     * Set the alpha of the layer, which will be concatenated to the current animation opacity for
+     * displaying.
+     */
+    public native void setAlpha(float value);
+
     private native void nativeRelease();
 
     protected long nativeContext;

src/platform/android/JPAGLayer.cpp

@@ -297,4 +297,14 @@ PAG_API void Java_org_libpag_PAGLayer_setExcludedFromTimeline(JNIEnv* env, jobje
 
   pagLayer->setExcludedFromTimeline(value);
 }
+
+ PAG_API void Java_org_libpag_PAGLayer_setAlpha(JNIEnv* env, jobject thiz,
+                                                jfloat value){
+  auto pagLayer = GetPAGLayer(env, thiz);
+  if(pagLayer == nullptr){
+    return;
+  }
+
+  pagLayer->setAlpha(value);
+}
 }

src/platform/cocoa/PAGLayer.h

@@ -165,4 +165,10 @@ PAG_API @interface PAGLayer : NSObject
  */
 - (void)setExcludedFromTimeline:(BOOL)value;
 
+/**
+ * Set the alpha of the layer, which will be concatenated to the current animation opacity for
+ * displaying.
+ */
+- (void)setAlpha:(float)value;
+
 @end

src/platform/cocoa/PAGLayer.m

@@ -133,6 +133,10 @@ - (void)setExcludedFromTimeline:(BOOL)value {
   [(PAGLayerImpl*)_impl setExcludedFromTimeline:value];
 }
 
+- (void)setAlpha:(float)value{
+  [(PAGLayerImpl*)_impl setAlpha:value];
+}
+
 - (void)dealloc {
   [_impl release];
   [super dealloc];

src/platform/cocoa/private/PAGLayerImpl.h

@@ -72,4 +72,6 @@
 
 - (void)setExcludedFromTimeline:(BOOL)value;
 
+- (void)setAlpha:(float)value;
+
 @end

src/platform/cocoa/private/PAGLayerImpl.mm

@@ -254,4 +254,8 @@ - (void)setExcludedFromTimeline:(BOOL)value {
   _pagLayer->setExcludedFromTimeline(value);
 }
 
+- (void)setAlpha:(float)value {
+  _pagLayer->setAlpha(value);
+}
+
 @end