Correct uSamplerEnable uniform in ImGui shader; make sure to pop the unused imgui command before rendering

Makes gui.drawString somewhat work under OpenGL, but there's still something wrong when it's combined with other draw functions

src/BizHawk.Bizware.Graphics/ImGuiResourceCache.cs

@@ -164,7 +164,7 @@ void main()
 		public const string ImGuiPixelShader_gl = @"
 //opengl 3.0
 #version 130
-bool uSamplerEnable;
+uniform bool uSamplerEnable;
 uniform sampler2D uSampler0;
 
 in vec2 vTexcoord0;

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

@@ -281,6 +281,7 @@ namespace BizHawk.Bizware.Graphics
 					DrawImage(_stringOutput, 0, 0);
 				}
 
+				_imGuiDrawList._PopUnusedDrawCmd();
 				RenderInternal(width, height);
 				ResetDrawList();
 			}