Use GetAlpha() in more places inside CFX_ScanlineCompositor.

Many functions have code that do the same thing as GetAlpha(), so just
call that instead.

Change-Id: I3e518b9c123d93d9011238db9d9175d92a097435
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/65090
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fxge/dib/cfx_scanlinecompositor.cpp

diff --git a/core/fxge/dib/cfx_scanlinecompositor.cpp b/core/fxge/dib/cfx_scanlinecompositor.cpp
index 3473bc2..f8d4bd0 100644
--- a/core/fxge/dib/cfx_scanlinecompositor.cpp
+++ b/core/fxge/dib/cfx_scanlinecompositor.cpp
@@ -1344,12 +1344,8 @@
         src_alpha_scan++;
         continue;
       }
-      uint8_t src_alpha;
-      if (clip_scan) {
-        src_alpha = clip_scan[col] * (*src_alpha_scan++) / 255;
-      } else {
-        src_alpha = *src_alpha_scan++;
-      }
+      uint8_t src_alpha = GetAlpha(*src_alpha_scan, clip_scan, col);
+      ++src_alpha_scan;
       if (src_alpha == 0) {
         dest_scan += 4;
         continue;
@@ -1796,12 +1792,7 @@
       dest_scan += 4;
       continue;
     }
-    int src_alpha;
-    if (clip_scan) {
-      src_alpha = mask_alpha * clip_scan[col] / 255;
-    } else {
-      src_alpha = mask_alpha;
-    }
+    int src_alpha = GetAlpha(mask_alpha, clip_scan, col);
     uint8_t back_alpha = dest_scan[3];
     if (back_alpha == 0) {
       FXARGB_SETDIB(dest_scan, ArgbEncode(src_alpha, src_r, src_g, src_b));
@@ -1875,12 +1866,7 @@
       dest_scan += Bpp;
       continue;
     }
-    int src_alpha;
-    if (clip_scan) {
-      src_alpha = mask_alpha * clip_scan[col] / 255;
-    } else {
-      src_alpha = mask_alpha;
-    }
+    int src_alpha = GetAlpha(mask_alpha, clip_scan, col);
     if (src_alpha == 0) {
       dest_scan += Bpp;
       continue;
@@ -1927,12 +1913,7 @@
       dest_scan++;
       continue;
     }
-    int src_alpha;
-    if (clip_scan) {
-      src_alpha = mask_alpha * clip_scan[col] / 255;
-    } else {
-      src_alpha = mask_alpha;
-    }
+    int src_alpha = GetAlpha(mask_alpha, clip_scan, col);
     uint8_t back_alpha = *dest_scan;
     if (!back_alpha) {
       *dest_scan = src_alpha;
@@ -1955,12 +1936,7 @@
       dest_scan++;
       continue;
     }
-    int src_alpha;
-    if (clip_scan) {
-      src_alpha = mask_alpha * clip_scan[col] / 255;
-    } else {
-      src_alpha = mask_alpha;
-    }
+    int src_alpha = GetAlpha(mask_alpha, clip_scan, col);
     if (src_alpha) {
       *dest_scan = FXDIB_ALPHA_MERGE(*dest_scan, src_gray, src_alpha);
     }
@@ -1982,12 +1958,7 @@
       dest_alpha_scan++;
       continue;
     }
-    int src_alpha;
-    if (clip_scan) {
-      src_alpha = mask_alpha * clip_scan[col] / 255;
-    } else {
-      src_alpha = mask_alpha;
-    }
+    int src_alpha = GetAlpha(mask_alpha, clip_scan, col);
     uint8_t back_alpha = *dest_alpha_scan;
     if (back_alpha == 0) {
       *dest_scan++ = src_gray;
@@ -2030,12 +2001,7 @@
       src_scan += 4;
       continue;
     }
-    uint8_t src_alpha;
-    if (clip_scan) {
-      src_alpha = clip_scan[col] * src_scan[3] / 255;
-    } else {
-      src_alpha = src_scan[3];
-    }
+    uint8_t src_alpha = GetAlpha(src_scan[3], clip_scan, col);
     if (src_alpha == 0) {
       dest_scan += 4;
       src_scan += 4;
@@ -2709,12 +2675,7 @@
       dest_scan += 4;
       continue;
     }
-    int src_alpha;
-    if (clip_scan) {
-      src_alpha = mask_alpha * clip_scan[col] / 255;
-    } else {
-      src_alpha = mask_alpha;
-    }
+    int src_alpha = GetAlpha(mask_alpha, clip_scan, col);
     uint8_t back_alpha = dest_scan[3];
     if (back_alpha == 0) {
       FXARGB_SETRGBORDERDIB(dest_scan,
@@ -2784,12 +2745,7 @@
       dest_scan += Bpp;
       continue;
     }
-    int src_alpha;
-    if (clip_scan) {
-      src_alpha = mask_alpha * clip_scan[col] / 255;
-    } else {
-      src_alpha = mask_alpha;
-    }
+    int src_alpha = GetAlpha(mask_alpha, clip_scan, col);
     if (src_alpha == 0) {
       dest_scan += Bpp;
       continue;