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/rmodels.c

@@ -1282,6 +1282,8 @@ void UploadMesh(Mesh *mesh, bool dynamic)
 
 #if defined(GRAPHICS_API_OPENGL_33) || defined(GRAPHICS_API_OPENGL_ES2)
     mesh->vaoId = rlLoadVertexArray();
+    if (mesh->vaoId == 0) return;
+
     rlEnableVertexArray(mesh->vaoId);
 
     // NOTE: Vertex attributes must be uploaded considering default locations points and available vertex data
@@ -1470,6 +1472,8 @@ void DrawMesh(Mesh mesh, Material material, Matrix transform)
     // Bind shader program
     rlEnableShader(material.shader.id);
 
+    if (material.shader.locs == NULL) return;
+
     // Send required data to shader (matrices, values)
     //-----------------------------------------------------
     // Upload to shader material.colDiffuse