Aggressively unbind GL_PIXEL_UNPACK_BUFFER

Bug: 27186019

Theory: It appears to be possible for FontRenderer
to not unbind its PBO prior to textures being uploaded,
resulting in trying to glSubTexImage2D with a bound
GL_PIXEL_UNPACK_BUFFER. In that scenario the void* is
the offset into the PBO which given a non-null data
will almost certainly overrun the end of the buffer. This
in turn produces a GL_INVALID_OPERATION error.

Change PixelBuffer to avoid leaking this state for now.
This will result in more calls to glBindBuffer/glUnbindBuffer
in the worst case, but the worst case is already bad so this
shouldn't be a problem. In the normal case we avoid binding
the PBO at all ever, so this doesn't impact that.

Change-Id: I05473f0d2f9a3a5da0e33d8f9ddea4731ce970e3
diff --git a/libs/hwui/PixelBuffer.h b/libs/hwui/PixelBuffer.h
index aac5ec4..bbef36b 100644
--- a/libs/hwui/PixelBuffer.h
+++ b/libs/hwui/PixelBuffer.h
@@ -91,14 +91,6 @@
     virtual uint8_t* map(AccessMode mode = kAccessMode_ReadWrite) = 0;
 
     /**
-     * Unmaps this buffer, if needed. After the buffer is unmapped,
-     * the pointer previously returned by map() becomes invalid and
-     * should not be used. After calling this method, getMappedPointer()
-     * will always return NULL.
-     */
-    virtual void unmap() = 0;
-
-    /**
      * Returns the current access mode for this buffer. If the buffer
      * is not mapped, this method returns kAccessMode_None.
      */
@@ -204,6 +196,14 @@
             mFormat(format), mWidth(width), mHeight(height), mAccessMode(kAccessMode_None) {
     }
 
+    /**
+     * Unmaps this buffer, if needed. After the buffer is unmapped,
+     * the pointer previously returned by map() becomes invalid and
+     * should not be used. After calling this method, getMappedPointer()
+     * will always return NULL.
+     */
+    virtual void unmap() = 0;
+
     GLenum mFormat;
 
     uint32_t mWidth;