Fix compilation warnings and re-enable werror=yes on Travis

Fix -Wunused-variable, -Wunused-but-set-variable and -Wswitch warnings
raised by GCC 8 and 9.

Fix -Wunused-function, -Wunused-private-field and
-Wtautological-constant-out-of-range-compare raised by Clang.

Fix MSVC 2019 warning C4804 (unsafe use of type 'bool' in comparison
operation).

GCC -Wcpp warnings/Clang -W#warnings (`#warning`) are no longer raising
errors and will thus not abort compilation with `werror=yes`.

Treat glslang headers are system headers to avoid raising warnings.

Re-enables us to build with `werror=yes` on Linux and macOS, thus
catching warnings that would be introduced by new code.

Fixes #36132.

servers/visual/rasterizer_rd/rasterizer_canvas_rd.cpp

@@ -991,6 +991,15 @@ void RasterizerCanvasRD::_render_item(RD::DrawListID p_draw_list, const Item *p_
 				}
 
 			} break;
+			case Item::Command::TYPE_MESH:
+			case Item::Command::TYPE_MULTIMESH:
+			case Item::Command::TYPE_PARTICLES: {
+				ERR_PRINT("FIXME: Mesh, MultiMesh and Particles render commands are unimplemented currently, they need to be ported to the 4.0 rendering architecture.");
+#ifndef _MSC_VER
+#warning Item::Command types for Mesh, MultiMesh and Particles need to be implemented.
+#endif
+				// See #if 0'ed code below to port from GLES3.
+			} break;
 
 #if 0
 			case Item::Command::TYPE_MESH: {