Add framebuffer attribute to main frame (#1131)

Mostly for Rae's benefit and whoever else is now deciding to alpha-ify
our main frame when moving.

As our main frame is a composition of various texture regions, adjusting
the alpha of the frame has a few ugly side effects where individual
textures representing the borders and sidebar divider end up blending
together.

This can be resolved by just enabling the framebuffer attribute on the
frame which will force the frame to be rendered to a target at full
opacity (effectively, resetting its effective alpha) before being
composited into the final UI scene with transparency applied.

Net result - none of the borders end up blending together. As an extra
add, as Keyboard pointed out our sidebar texture was smaller than its
actual space so the anchors have been adjusted slightly to compensate.

totalRP3/UI/Main.xml

@@ -205,7 +205,7 @@ https://raw.githubusercontent.com/Meorawr/wow-ui-schema/main/UI.xsd">
 		</Frames>
 	</Frame>
 
-	<Frame name="TRP3_MainFrame" inherits="TRP3_MainFrameTemplate" mixin="TRP3_MainFrameLayoutMixin" hidden="true" movable="true" dontSavePosition="true">
+	<Frame name="TRP3_MainFrame" inherits="TRP3_MainFrameTemplate" mixin="TRP3_MainFrameLayoutMixin" hidden="true" movable="true" dontSavePosition="true" frameBuffer="true">
 		<Anchors>
 			<Anchor point="CENTER" x="0" y="0"/>
 		</Anchors>
@@ -248,6 +248,10 @@ https://raw.githubusercontent.com/Meorawr/wow-ui-schema/main/UI.xsd">
 					<Layer level="BACKGROUND">
 						<Texture parentKey="Background" file="Interface\AddOns\totalRP3\Resources\UI\ui-frame-neutral-background" horizTile="true" vertTile="true">
 							<Color r="0.6" g="0.6" b="0.6"/>
+							<Anchors>
+								<Anchor point="TOPLEFT" x="-8" y="8"/>
+								<Anchor point="BOTTOMRIGHT" x="8" y="-8"/>
+							</Anchors>
 						</Texture>
 					</Layer>
 					<Layer level="BORDER" textureSubLevel="-3">