drm_hwcomposer: Use float type to store alpha internally

0.0-1.0F range is a natural way to store alpha value internally,
to be converted later to DRM-specific 0-0xFFFF fixed point type at
DRM layer.

Change-Id: Iedf2b11e53bf38efd08cd4eda1346647b9ff721f
Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
diff --git a/compositor/LayerData.h b/compositor/LayerData.h
index 127b3ff..962141f 100644
--- a/compositor/LayerData.h
+++ b/compositor/LayerData.h
@@ -60,9 +60,11 @@
   std::optional<IRect> i_rect;
 };
 
+constexpr float kAlphaOpaque = 1.0F;
+
 struct PresentInfo {
   LayerTransform transform{};
-  uint16_t alpha = UINT16_MAX;
+  float alpha = kAlphaOpaque;
   SrcRectInfo source_crop{};
   DstRectInfo display_frame{};