Fix drawString not blending when it should

Drawing a string is really having GDI+ do the drawing in software, uploading the result to a texture, then overlaying it to the render target. Since this texture is largely transparent, it needs blending. Of course, assuming the colors are all otherwise opaque, we only need to do this for the string texture draw

src/BizHawk.Bizware.Graphics/Renderers/ImGui2DRenderer.cs

@@ -171,9 +171,9 @@ namespace BizHawk.Bizware.Graphics
 			DrawString,
 		}
 
-		private void PerformPasses(ImDrawCmdPtr cmd)
+		private void PerformPasses(ImDrawCmdPtr cmd, bool forceBlending)
 		{
-			if (EnableBlending && _needBlending)
+			if ((EnableBlending && _needBlending) || forceBlending)
 			{
 				_pass1RenderTarget.Bind();
 				_resourceCache.SetBlendingParamters(_pass2RenderTarget, false);
@@ -249,7 +249,7 @@ namespace BizHawk.Bizware.Graphics
 							_resourceCache.SetTexture(null);
 						}
 
-						PerformPasses(cmd);
+						PerformPasses(cmd, forceBlending: false);
 						tempTex?.Dispose();
 						break;
 					}
@@ -281,7 +281,11 @@ namespace BizHawk.Bizware.Graphics
 
 							_stringTexture.LoadFrom(new BitmapBuffer(_stringOutput, new()));
 							_resourceCache.SetTexture(_stringTexture);
-							PerformPasses(lastCmd);
+
+							// the string texture is largely transparent, so it implicitly needs blending
+							// however, other things might not need blending
+							// so as an optimization, don't force blending everywhere, just here if need be
+							PerformPasses(lastCmd, forceBlending: true);
 							ClearStringOutput();
 						}
 
@@ -418,11 +422,11 @@ namespace BizHawk.Bizware.Graphics
 				throw new InvalidOperationException("Invalid number of points");
 			}
 
-			CheckAlpha(color);
 			var startPt = points[0];
 			var col = (uint)color.ToArgb();
 			for (var i = 1; i < points.Length; i += 3)
 			{
+				CheckAlpha(color);
 				_imGuiDrawList.AddBezierCubic(
 					p1: startPt.ToVector(),
 					p2: points[i + 0].ToVector(),
@@ -620,6 +624,10 @@ namespace BizHawk.Bizware.Graphics
 		public void DrawString(string s, Font font, Color color, float x, float y, StringFormat format, TextRenderingHint textRenderingHint)
 		{
 			_splitNextCmd = false;
+			// drawing a string in the end just overlays a largely transparent texture with the drawn text
+			// as such, it implicitly needs blending during the string drawing
+			// however, as an optimization, we can avoid blending in general if the string color is opaque
+			// if it isn't opaque, we need to blend everywhere regardless
 			_needBlending |= color.A != 0xFF;
 			var stringArgs = new DrawStringArgs { Str = s, Font = font, Color = color, X = x, Y = y, Format = format, TextRenderingHint = textRenderingHint };
 			var handle = GCHandle.Alloc(stringArgs, GCHandleType.Normal);

src/BizHawk.Bizware.Graphics/Renderers/SDLImGui2DRenderer.cs

@@ -209,11 +209,22 @@ namespace BizHawk.Bizware.Graphics
 
 									try
 									{
+										// have to blend here, due to transparent texture usage
+										if (!EnableBlending)
+										{
+											_ = SDL_SetRenderDrawBlendMode(sdlRenderer, SDL_BlendMode.SDL_BLENDMODE_BLEND);
+										}
+
 										RenderCommand(sdlRenderer, sdlTex, _imGuiDrawList, lastCmd);
 									}
 									finally
 									{
 										SDL_DestroyTexture(sdlTex);
+
+										if (!EnableBlending)
+										{
+											_ = SDL_SetRenderDrawBlendMode(sdlRenderer, SDL_BlendMode.SDL_BLENDMODE_NONE);
+										}
 									}
 								}
 								finally