Add OSD background support for the static portions of display elements

Significantly reduces the time the OSD task spends drawing elements that are completely or mostly static. The larger the element the more time savings are realized. Currently implemented support for:
- Crosshairs
- Artificial Horizon Sidebars
- Craft name
- Display name
- Stick overlay

Since the static portions are only rendered once, the static elements add no processing time to the OSD task. As an example, enabling the above elements prior to these changes results in a total rendering time of 47us. After the enhancements they take only 6us (basically the rendering phase minimum overhead). So effectively 41us are removed from the OSD task.

Opens the possibility to add large mostly static elements with no additional overhead. An example would be a camera framing element that might draw a "box" around most of the screen. Previously this would add significant processing overhead to the OSD task, but now it will have no impact.

src/main/cms/cms.c

@@ -720,6 +720,7 @@ void cmsMenuOpen(void)
         currentCtx = (cmsCtx_t){ &menuMain, 0, 0 };
         menuStackIdx = 0;
         setArmingDisabled(ARMING_DISABLED_CMS_MENU);
+        displayLayerSelect(pCurrentDisplay, DISPLAYPORT_LAYER_FOREGROUND); // make sure the foreground layer is active
     } else {
         // Switch display
         displayPort_t *pNextDisplay = cmsDisplayPortSelectNext();