Fix stencil color mask;

- When calling lovr.graphics.stencil, the color mask is initially
  disabled, and gets restored to its initial state afterwards.
- However, when it's restored, it uses lovrGraphicsSetColorMask, which
  just sets shadow state that doesn't make it all the way to GL until
  another draw is done.
- The consequence of this is that if you call .stencil and then don't do
  a draw, any clears that happen will use the old (disabled) color mask,
  preventing the color buffer from being cleared.
- The solution here is to lower the color mask change down into opengl.c
  where it can directly hit OpenGL.

src/modules/graphics/opengl.c

@@ -1500,6 +1500,11 @@ void lovrGpuPresent() {
 
 void lovrGpuStencil(StencilAction action, int replaceValue, StencilCallback callback, void* userdata) {
   lovrGraphicsFlush();
+
+  uint8_t lastColorMask = state.colorMask;
+  state.colorMask = 0;
+  glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
+
   if (!state.stencilEnabled) {
     state.stencilEnabled = true;
     glEnable(GL_STENCIL_TEST);
@@ -1524,6 +1529,9 @@ void lovrGpuStencil(StencilAction action, int replaceValue, StencilCallback call
   lovrGraphicsFlush();
   state.stencilWriting = false;
   state.stencilMode = ~0; // Dirty
+
+  state.colorMask = lastColorMask;
+  glColorMask(state.colorMask & 0x8, state.colorMask & 0x4, state.colorMask & 0x2, state.colorMask & 0x1);
 }
 
 void lovrGpuDirtyTexture() {