Merge vram dirty tracking

Squashed commit of the following:

commit b463a05d4b909372f0cd1ad91caa0c77a25e5901
Author: RSDuck <rsduck@users.noreply.github.com>
Date:   Mon Nov 30 01:55:35 2020 +0100

    minor fix

commit ce73cebbdf5da243d7ebade82d8799ded9cd6b28
Author: RSDuck <rsduck@users.noreply.github.com>
Date:   Mon Nov 30 00:43:08 2020 +0100

    fix dirty flags of BG/OBJ mappings not being reset

commit fc5d73a6178e3adc444398bdd23de8314b5ca8f8
Author: RSDuck <rsduck@users.noreply.github.com>
Date:   Mon Nov 30 00:11:13 2020 +0100

    use flat vram for gpu2d everywhere

commit 34ee9fe2bf04fcfa2a5a1c8d78d70007e606f1a2
Author: RSDuck <rsduck@users.noreply.github.com>
Date:   Sat Nov 28 19:10:34 2020 +0100

    mark VRAM dirty for display capture

commit e8778fa2f429c6df0eece19d6a5ee83ae23a0cf4
Author: RSDuck <rsduck@users.noreply.github.com>
Date:   Sat Nov 28 18:59:31 2020 +0100

    use flat VRAM for textures and texpals
    also skip rendering if nothing changed and a bunch of fixes

commit 53f2041e2e1a28b35702a2ed51de885c36689f71
Author: RSDuck <rsduck@users.noreply.github.com>
Date:   Fri Nov 27 18:29:56 2020 +0100

    use vram dirty tracking for extpals
    also preparations to take this further

commit 4cdfa329e95aed26d3b21319c8fd86a04abf20f7
Author: RSDuck <rsduck@users.noreply.github.com>
Date:   Mon Nov 16 23:32:22 2020 +0100

    VRAM dirty tracking

1 files changed, 2 insertions, 0 deletions

diff --git a/src/GPU3D.h b/src/GPU3D.h
index c69adde..0477c4f 100644
--- a/src/GPU3D.h
+++ b/src/GPU3D.h
@@ -87,6 +87,8 @@ extern u8 RenderFogDensityTable[34];
 
 extern u32 RenderClearAttr1, RenderClearAttr2;
 
+extern bool RenderFrameIdentical;
+
 extern std::array<Polygon*,2048> RenderPolygonRAM;
 extern u32 RenderNumPolygons;