minui: Move GRSurface into a class.

This CL adds GRSurface::Create() and dtor for managing the allocated
memory in GRSurface class. It also adds GRSurface::data() that hides the
underlying implementation, with both of const and non-const overloads.
This allows `const GRSurface&` to be more useful - previously it only
ensured a const member variable of `data`, instead of a read-only buffer
it points to.

It also marks the parameters in gr_texticon() and gr_blit() as const, as
they're incoming source that shouldn't be altered. It corrects the type
of gr_draw, which is the sink to be painted on (an earlier attempt was
made in [1], but didn't get the full picture correctly).

[1] https://android-review.googlesource.com/c/platform/bootable/recovery/+/704757/

Test: mmma -j bootable/recovery
Test: recovery_unit_test on marlin
Test: Run graphics test on marlin (fbdev).
Test: Run graphics test on blueline (drm).
Change-Id: I7904df084cd6c08fa04a9da97d01b4b1a6e3a20c
diff --git a/minui/graphics_fbdev.h b/minui/graphics_fbdev.h
index 107e195..be813dc 100644
--- a/minui/graphics_fbdev.h
+++ b/minui/graphics_fbdev.h
@@ -14,14 +14,27 @@
  * limitations under the License.
  */
 
-#ifndef _GRAPHICS_FBDEV_H_
-#define _GRAPHICS_FBDEV_H_
+#pragma once
 
 #include <linux/fb.h>
+#include <stdint.h>
 
 #include "graphics.h"
 #include "minui/minui.h"
 
+class GRSurfaceFbdev : public GRSurface {
+ public:
+  uint8_t* data() override {
+    return buffer_;
+  }
+
+ private:
+  friend class MinuiBackendFbdev;
+
+  // Points to the start of the buffer: either the mmap'd framebuffer or one allocated in-memory.
+  uint8_t* buffer_;
+};
+
 class MinuiBackendFbdev : public MinuiBackend {
  public:
   GRSurface* Init() override;
@@ -33,12 +46,10 @@
  private:
   void SetDisplayedFramebuffer(unsigned n);
 
-  GRSurface gr_framebuffer[2];
+  GRSurfaceFbdev gr_framebuffer[2];
   bool double_buffered;
-  GRSurface* gr_draw;
+  GRSurfaceFbdev* gr_draw;
   int displayed_buffer;
   fb_var_screeninfo vi;
   int fb_fd;
 };
-
-#endif  // _GRAPHICS_FBDEV_H_