REVIEWED: Avoid program crash if GPU data is tried to be loaded before InitWindow() #4751

Following raylib design, a warning log message is shown and program can continue execution.
Some early return checks have been added on most  critical functions.
[rtext] Previous implementation checking `isGpuReady` cross-module variable is not needed any more, resulting in a more decoupled code, load failure is managed at rlgl level

src/platforms/rcore_android.c

@@ -84,7 +84,6 @@ typedef struct {
 // Global Variables Definition
 //----------------------------------------------------------------------------------
 extern CoreData CORE;                   // Global CORE state context
-extern bool isGpuReady;                 // Flag to note GPU has been initialized successfully
 static PlatformData platform = { 0 };   // Platform specific data
 
 //----------------------------------------------------------------------------------
@@ -1042,7 +1041,6 @@ static void AndroidCommandCallback(struct android_app *app, int32_t cmd)
                     // Initialize OpenGL context (states and resources)
                     // NOTE: CORE.Window.currentFbo.width and CORE.Window.currentFbo.height not used, just stored as globals in rlgl
                     rlglInit(CORE.Window.currentFbo.width, CORE.Window.currentFbo.height);
-                    isGpuReady = true;
 
                     // Setup default viewport
                     // NOTE: It updated CORE.Window.render.width and CORE.Window.render.height